First-Trimester Ultrasound: A Comprehensive Guide

9. Fetal Biometry in Early Pregnancy

Lea M. Porche Steven Warsof1 and Alfred Abuhamad1

(1)

Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, Eastern Virginia Medical School, 825 Fairfax Ave., Suite 544, Norfolk, VA 23507, USA

Lea M. Porche

Email: porchelm@evms.edu

Keywords

BiometryCrown-rump lengthCystic hygromaGestational ageGestational sacNasal boneNuchal translucencyYolk sac

Introduction

Fetal biometry, morphometric measurements of the fetus and gestational structures, has routinely been utilized in the second trimester for the determination of gestational age, estimation of abnormalities in fetal growth and weight, and determination of normal and abnormal fetal anatomy. With improvements in ultrasound technology, biometry in the first trimester has become a more accurate and useful tool. Many structures can be measured in the first trimester and are now able to give clues regarding pregnancy location, viability, gestational age, chorionicity in multiple gestations, and the risk of aneuploidy. In this chapter, we review the assessment of these structures and their significance in the first trimester.

Biometry

Gestational Sac

As the earliest sonographic evidence of intrauterine pregnancy, the gestational sac (GS) can be seen as early as 4 weeks of gestation, just days after the first missed menses [1]. The GS can also be used in early pregnancy for assessment of dating [2]. When seen at 4 weeks, the gestational sac is about 2–3 mm in diameter and in this early phase grows rapidly at a rate of about 1 mm per day [3] (Table 9.1).

Table 9.1

Relation between mean sac diameter (MSD) and menstrual agea

Mean sac diameter (mm)

Predicted age range (weeks) = 95 % CI

2

5.0 (4.5–5.5)

3

5.1 (4.6–5.6)

4

5.2 (4.8–5.7)

5

5.4 (4.9–5.8)

6

5.5 (5.0–6.0)

7

5.6 (5.1–6.1)

9

5.9 (5.4–6.3)

10

6.0 (5.5–6.5)

11

6.1 (5.6–6.6)

12

6.2 (5.8–6.7)

13

6.4 (5.9–6.8)

14

6.5 (6.0–7.0)

15

6.6 (6.2–7.1)

16

6.7 (6.3–7.2)

17

6.9 (6.4–7.3)

18

7.0 (6.5–7.5)

19

7.1 (6.6–7.6)

20

7.3 (6.8–7.7)

21

7.4 (6.9–7.8)

22

7.5 (7.0–8.0)

23

7.6 (7.2–8.1)

24

7.8 (7.3–8.3)

aAdapted from Daya S, Wood S, Ward S, Lappalainen R, Caco C. Early pregnancy assessment with transvaginal ultrasound scanning. Can Med Assoc J 144:441, 1991

One measurement that has been used with high accuracy is the mean sac diameter (MSD). This measurement is obtained by taking the average of the measurements of the GS in three planes: coronal, sagittal, and transverse [1]. The MSD is useful early in the first trimester, but loses accuracy when it becomes greater than 14 mm, at which time the fetal pole should become visible. When measuring the dimensions of the GS, calipers should be placed on its borders and care should be taken to avoid including the surrounding decidual tissue [4] (Fig. 9.1).

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Fig. 9.1

The fetal pole and yolk sac can be seen within this gestational sac. Caliper measurements of the gestational sac are taken in the coronal and transverse planes. A third measurement will be taken in the sagittal plane to complete the three required measurements. The fetal crown-rump length will be used for the most accurate dating

Caution must be exercised in differentiating a true gestational sac from a pseudosac or a small intrauterine fluid or blood collection, both of which can be associated with ectopic or failed pregnancies [1] (Fig. 9.2). A true GS should typically be located eccentrically within the endometrial cavity due to it being embedded within the decidual layer [1]. There should also be evidence of the “double ring” sign, which refers to two echogenic rings surrounding the gestational sac. These rings represent the chorionic cavity with its associated villi and the surrounding developing decidua [5] (Fig. 9.3). If eccentric location of the GS with a double ring sign are not seen in a woman with a positive pregnancy test, a viable intrauterine pregnancy cannot be excluded, but these findings should raise suspicion for abnormal or extrauterine pregnancy, and close clinical follow-up is indicated [6].

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Fig. 9.2

This fluid collection within the uterus is a “pseudo sac” in the setting of an abdominal pregnancy. Note the central location and absence of two echogenic rings. These characteristics help to distinguish this from a true gestational sac associated with viable intrauterine pregnancy

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Fig. 9.3

“Double ring sign.” The gestational sac can be seen with the eccentrically located fetal pole measured within. The two echogenic rings surrounding the gestational sac are clear in this image

Yolk Sac

The yolk sac (YS) first appears within the GS at 5 weeks of gestation, and is frequently the first identifiable structure within the GS [178]. Functioning as the first nutritional and metabolic support for the developing embryo prior to establishment of the placenta, it also offers ultrasonographic confirmation of intrauterine pregnancy [9] (Fig. 9.4).

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Fig. 9.4

Normal yolk sac. The yolk sac (above) can be seen in close proximity to the fetal pole (below). The hypoechoic developing rhombencephalon can be seen at the right end of the fetal pole

While usually apparent by week 5 of gestation, the YS may not be visible until later, when the MSD is closer to 8 mm [7]. It is connected to the embryo by the vitelline duct. When the amnion forms around the fetus, the YS is then seen as an extra-amniotic structure. The YS progresses in size to a usual maximum of 6 mm around 10 weeks, and then regresses until it is absorbed between the amnion and chorion by the completion of week 12–13 [10]. Measurement of the YS should be performed by placing the calipers on the innermost border of the echogenic rim [11].

Nomograms relating YS size with gestational age have been developed [12], but due to marked variation in normal pregnancies, YS diameter should not be used a primary means of pregnancy dating [10].

As mentioned previously, the YS offers confirmation of intrauterine pregnancy, and can even help to indicate amnionicity in multiple gestations, as the number of YSs should correlate with the number of amniotic sacs if the embryos are viable [8]. This can be particularly important in higher order multiple gestations.

Marked variation in the size and shape of the YS can be noted. These variations may be of significance. A small or large YS (<3 mm prior to 6–10 weeks and >7 mm prior to 9 weeks) may be suspicious for an abnormally developing pregnancy. These cases should be followed up with repeat ultrasound evaluation to confirm progression of the pregnancy [1]. Absence of the YS or embryo in the presence of a MSD of ≥25 mm is diagnostic of a failed pregnancy with specificity and positive predictive value approaching 100 % [12]. Echogenic, irregularly shaped, or persistent YS, particularly after 12 weeks gestation, are of uncertain significance [13].

Crown-Rump Length

The fetal pole is first visible by transvaginal ultrasound at 5 weeks gestation with cardiac activity notable by 6–6.5 weeks gestation [1]. It is important to note that fetal heart rates can be slower than anticipated in these very early pregnancies, but should be within the normal range by 8 weeks gestation.

The first true fetal biometric measurement possible is the crown-rump length (CRL). By definition, the CRL is not actually measured from the fetal crown to its rump, but instead the longest linear dimension from the cephalic to the caudal end of the embryo with the fetus in neutral position (Fig. 9.5). In early gestation, between 6 and 9 weeks, fetal posture makes little difference in the CRL measurement, but beyond this point, flexion or extension can cause significant discrepancy.

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Fig. 9.5

Crown-rump length. Here the calipers are placed at the cephalic and caudal ends of the fetus. This fetus appears to be slightly flexed at the time of measurement

Obtaining the CRL should be done in a standardized fashion to increase the accuracy of the measurement. A midsagittal section of the embryo should be captured and the image maximized to fill the majority of the screen. Care should be taken to attempt capturing this image with the embryo in neutral position, avoiding hyperflexion or extension. The two ends of the embryo should be well defined, and the caliper function on the ultrasound machine used to capture the measurement. In extremely early gestations, the cephalic and caudal ends of the fetus may not be distinguishable. In this scenario, the greatest longitudinal measurement should be obtained [14] (Fig. 9.6).

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Fig. 9.6

Crown-rump length. This early crown-rump length measurement demonstrates the difficulty in identifying cephalic and caudal ends of the embryo at this early gestational age. Here, the greatest longitudinal measurement is obtained. The yolk sac can be seen in close proximity to the fetal pole

The primary importance of the CRL measurement is in pregnancy dating. One of the first to pursue the biometric measurement of the fetal pole was Dr. Hugh Robinson, who worked with Professor Ian Donald at The Queen Mother’s Hospital in Glasgow, Scotland. In early 1970s, he published works that gave validity to the use of ultrasound in the measurement of the early fetal pole. In one study, he evaluated women between 6 and 14 weeks gestation with regular cycles and known last menstrual periods by B-mode transabdominal ultrasound techniques [15]. He plotted his measurements against menstrual age, and in those with missed abortion, against the physical measurement of the conceptus after delivery. He noted a high degree of correlation between ultrasound measurements and menstrual age. Despite the most rudimentary of ultrasound equipment, his meticulous measurements have stood the test of time and are still used over 40 years later. Hence, first-trimester ultrasound with measurement of CRL is now considered the most reliable method of pregnancy dating with known and unknown last menstrual period. More recently, the accuracy of transvaginal ultrasound as a means for pregnancy dating was confirmed in a study by Pexters et al. showing that in 54 patients, CRL and MSD measurements showed high interobserver and intraobserver correlation, and were highly reproducible [16].

Many studies have been completed in different populations to assess the ability to generalize these initial nomograms. One such study was performed by Papageorghiou et al. in eight geographically different countries. Data from 4265 women were included to determine an equation that would be generalizable to multiple populations [17]. Many nomograms for CRL have been developed over the years. Based on the population, prediction equations can differ significantly. For example, the CRL curves developed by Robinson and Pexsters differ at very early gestations, but are very similar after about 8 weeks. Most published CRL curves differ very little from the measurement published by Dr. Robinson in 1973 (Tables 9.2 and 9.3).

Table 9.2

Gestational age estimation by crown-rump length (CRL): Robinsona

Fetal CRL (mm)

Gestational age (weeks + days)

5

6 + 0

10

7 + 1

15

7 + 6

20

8 + 4

25

9 + 2

30

9 + 6

35

10 + 2

40

10 + 6

45

11 + 2

50

11 + 5

55

12 + 1

60

12 + 3

65

12 + 6

70

13 + 1

75

13 + 4

80

13 + 6

85

14 + 1

Formula

GA (days) = 8.052 × (CRL × 1.037)1/2 + 23.73

aAdapted from Robinson HP, Fleming JE. A critical evaluation of sonar “crown-rump length” measurements. Br J Obstet Gynaecol 1975; 82:702–10

Table 9.3

Gestational age estimation by crown-rump length (CRL): Pexstersa

Mean CRL (mm)

Gestational age (weeks + days)

0.4

5 + 5

1.1

5 + 6

1.9

6 + 0

2.7

6 + 1

3.5

6 + 2

4.3

6 + 3

5.2

6 + 4

6.1

6 + 5

7.0

6 + 6

8.0

7 + 0

8.9

7 + 1

9.9

7 + 2

10.9

7 + 3

12.0

7 + 4

13.1

7 + 5

14.2

7 + 6

15.3

8 + 0

16.4

8 + 1

17.6

8 + 2

18.8

8 + 3

20.0

8 + 4

21.2

8 + 5

22.5

8 + 6

23.8

9 + 0

25.1

9 + 1

26.4

9 + 2

27.8

9 + 3

29.2

9 + 4

30.6

9 + 5

32.0

9 + 6

33.5

10 + 0

35.0

10 + 1

36.5

10 + 2

38.1

10 + 3

39.6

10 + 4

41.2

10 + 5

42.8

10 + 6

44.5

11 + 0

46.1

11 + 1

47.8

11 + 2

49.5

11 + 3

51.3

11 + 4

53.0

11 + 5

54.8

11 + 6

56.6

12 + 0

58.5

12 + 1

60.3

12 + 2

62.2

12 + 3

64.1

12 + 4

66.1

12 + 5

68.0

12 + 6

70.0

13 + 0

72.0

13 + 1

74.0

13 + 2

76.1

13 + 3

78.2

13 + 4

80.3

13 + 5

82.4

13 + 6

84.6

14 + 0

aAdapted from Pexsters A, Daemen A, Bottomley C, Van Schoubroeck D, De Catte L, De Moor B, et al. New crown-rump length curve based on over 3500 pregnancies. Ultrasound Obstet Gynecol 2010; 35: 650–655

Measurement of CRL can routinely be completed via transvaginal ultrasound by 6 weeks of gestation. When measured between weeks 7 and 10, CRL is proven to be accurate within 3 days of actual gestational age [1518]. However between 10 and 14 weeks, the accuracy decreases slightly to a margin of ±5 days [19], and with the addition of just one more week, the accuracy at 15 weeks gestation is as wide as ±8 days [20]. This reinforces the fact that for the most accurate pregnancy dating, CRL should be measured between 7 and 10 weeks of gestation. Of note, once the CRL measures beyond 84 mm (about 14 weeks gestation) the biparietal diameter (BPD) has been proven to be more accurate in pregnancy dating [15]. While many complex formulas have been determined, an easy formula to correlate gestational age with CRL from 7 to 14 weeks gestation is:

 $$ \mathrm{G}\mathrm{A}\left(\mathrm{weeks}\right)=6.5+\mathrm{C}\mathrm{R}\mathrm{L}\left(\mathrm{cm}\right) $$

When assigning a due date for early pregnancy, the American College of Obstetricians and Gynecologists (ACOG) have published criteria regarding what degree of discrepancy warrants a change in assigned due date. In the first trimester prior to 9 weeks gestation, the due date should be reassigned if the discrepancy between the ultrasound and menstrual dating is ±5 days. Between 9 and 15 + 6 weeks, dating should be reassigned based on a discrepancy of ±7 days [2122] (Table 9.4).

Table 9.4

Guidelines for redating pregnancy based on ultrasound in first trimestera

Gestational age range (weeks + days)

Method of measurement

Discrepancy between ultrasound dating and LMP dating that supports redating

≤13 + 6

CRL

 

• ≤8 + 6

More than 5 days

• 9 + 0–13 + 6

More than 7 days

14 + 0–15 + 6

BPD, HC, AC, FL

More than 7 days

16 + 0–21 + 6

BPD, HC, AC, FL

More than 10 days

aAdapted from ACOG Committee Opinion 611: Method for Estimating Due Date, October 2014

Nuchal Translucency

The importance of the nuchal translucency (NT) measurement in fetal medicine was first recognized by the pioneering work of Professor Kypros Nicolaides in the mid 1990s at King’s College Hospital in London, UK. Measurement of the nuchal translucency is now recommended as an option for patients as a part of first trimester screening for aneuploidy [23]. One element of the first trimester screening exam, the NT measurement is combined with levels of maternal serum beta human chorionic gonadotropin (β-hCG) and pregnancy-associated plasma protein-A (PAPP-A) [24]. These parameters together with maternal age give a patient-specific risk for trisomy 21 and 18. For trisomy 21, the detection rate is 85 % with a 5 % false-positive rate, which is higher than the detection rate in the second trimester using multiple maternal serum markers alone [25].

The NT is a hypoechoic structure located under the skin on the posterior fetal neck that represents fluid collection in that space [1] (Fig. 9.7). This structure can be identified and measured in all normal pregnancies, but the measurement is increased in cases of fetal aneuploidy or congenital heart disease. In monochorionic twins intertwin discrepancies in the NT measurement have been associated with early evidence of twin–twin transfusions syndrome [26].

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Fig. 9.7

Normal NT measurement. Here a normal NT measurement can be seen with all criteria met. Note the amnion that is clearly seen as separate from the posterior margin of the nuchal fluid collection

There are multiple theories regarding the etiology of increased NT measurements. In trisomy 21, dermal collagen has more hydrophilic properties, trapping fluid in the subcutaneous tissues [3]. In Turner syndrome, dysplastic lymphatics are credited with obstruction of the normal flow of fluid out of this space. Abnormal lymphatic drainage can also arise in the absence of Turner syndrome, leading to increased NT, enlarged jugular venous sacs and subsequent increase in venous pressure that can be detected as decreased or absent end diastolic flow in the ductus venosus [2728]. Finally, it has been seen that an enlarged NT can be associated with congenital cardiac disease, especially septal defects. It is postulated that endothelial dysfunction is responsible for the concurrent appearance of these two abnormalities. Importantly, it has not been proven that an enlarged NT measurement is a sign of cardiac failure and it should not be considered a marker for hydrops [29] (Fig. 9.8).

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Fig. 9.8

Enlarged NT measurement. The enlarged NT can be well seen in this image. Note that the measurement is taken at the largest portion of the fluid collection

Similarly, a cystic hygroma arises from obstruction of lymphatic flow into the venous system, leading to distention of the jugular venous sacs. Depending on the size of the cystic hygroma, it may be difficult to differentiate from an enlarged NT. While an enlarged NT is usually confined to the cervical region, cystic hygromas are usually larger and extend beyond the neck. They also often contain septations that make their appearance differ from that of an enlarged NT [30] (Fig. 9.9a, b). Care should be taken not to confuse posterior neural tube defects with a cystic hygroma, as they can be similar in appearance [27].

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Fig. 9.9

(a) Cystic hygroma: sagittal view of a fetus with a cystic hygroma. This fluid collection is not confined to the posterior cervical region, but instead extends cephalad to the face and caudad to the sacrum and legs. In this image, the fetal head is on the left, and the legs and pelvis are to the right. (b) Cystic hygroma: an axial view of the same cystic hygroma. The bones of the calvarium can be seen with surrounding increase in soft tissue and fluid. Posteriorly (right), fluid pockets and septations can be seen

The differential diagnosis for conditions associated with an enlarged NT can be seen in Table 9.5. Enlargement of the NT measurement is defined in most settings as an NT measurement above the 95th percentile for gestational age or ≥3 mm [23].

Table 9.5

Differential diagnosis for enlarged nuchal translucency (NT)a

Aneupoidy

• Trisomy 21

• Trisomy 13

• Trisomy 18

• Monosomy X

• Triploidy

Structural anomalies

• Cardiac defects

• Diaphragmatic hernia

• Renal anomalies

• Body stalk disruption

• Abdominal wall defects

Genetic syndromesb

• Noonan syndrome

• Roberts syndrome

• Cornelia de Lange syndrome

• Congenital adrenal hyperplasia

• Spinal muscular atrophy

• DiGeorge syndrome

• Smith–Lemli–Opitz syndrome

• Various skeletal dysplasias

Increased risk of twin-to-twin transfusion syndrome

aAdapted from Simpson LL. First trimester cystic hygroma and increased nuchal translucency, UpToDate 2014

bNot a comprehensive list

Accurate acquisition of the NT measurement is of great importance. In fact, no other ultrasound measurement requires the precision needed for accurate assessment of aneuploidy risk. The measurement should be obtained between 11 and 13 + 6 week gestation which is equivalent to a CRL of 45–84 mm [14]. Images can be obtained transvaginally or transabdominally using a high-resolution ultrasound machine. A magnified midsagittal section through the head and upper torso must be obtained and captured with the fetus in neutral position. The echogenic tip of the fetal nose is an indicator that one is imaging through this midsagittal plane. The amnion, which has not yet fused with the chorion at this gestation, should be visualized to ensure that the measurement is only of the NT and does not include intramniotic fluid. The calipers should be placed on the inner margins of the thickest portion of the NT and this is where the measurement should be obtained. If multiple adequate images are obtained, the largest measurement should be used for determination of risk [14]. The criteria needed to obtain an accurate NT measurement are shown in Table 9.6. With such extensive criteria, it is possible that an NT measurement cannot always be obtained. Some limiting factors are fetal position and maternal body habitus. If the NT cannot be obtained and first trimester screening cannot be completed, the patient should be offered alternative risk assessment for aneuploidy commensurate with her clinical scenario.

Table 9.6

Guidelines for measurement of nuchal translucency (NT)a

• Margins of NT clear enough for proper caliper placement

• Fetus in midsagittal plane

• Image magnified to be filled with fetal head, neck and upper thorax

• Fetal neck in neutral position

• Amnion must be seen separate from the NT

• Calipers must be used for measurement

• Calipers must be placed on the inner border of the nuchal line space with none of the horizontal crossbar protruding into the space

• Calipers placed perpendicular to the long axis of the fetus

• Measurement obtained at the widest space of the NT

aAdapted from the AIUM Practice Guideline for the performance of Obstetric Ultrasound Examinations, 2013

It is also important that a practice seeking to perform NT measurements is adequately equipped to acquire accurate images and to manage any abnormalities diagnosed. High-resolution ultrasound equipment should be available for use. Special training, certification, and maintenance of certification for those obtaining and interpreting the image are also required. Finally, appropriate counseling and follow up strategies should be in place to address abnormal results [14].

Follow-up of high-risk first trimester screening is of utmost importance in patient care. Genetic counseling should be made available so that patients can explore all of their options for genetic testing. For those wanting more definitive assessment of their risk, they can undergo a second screening test by evaluation of maternal cell-free DNA that yields a sensitivity of 99 % with a false-positive rate of 1 %. Late in the first trimester, prior to fusion of the chorion and amnion, the most common diagnostic test that is offered is chorionic villus sampling for direct evaluation of karyotype. Comparative genetic hybridization (CGH) studies can also be done to identify subchromosomal abnormalities at the same time. Later in the early second trimester, amniocentesis can be performed to obtain fetal cells for karyotype once the amnion and chorion have fused. If aneuploidy is unable to be ruled out with diagnostic testing, close ultrasound surveillance should be undertaken with detailed anatomic survey and fetal echocardiography in the second trimester [3]. Details on aneuploidy screening are available in Chap. 8.

Nasal Bone

The absence of the nasal bone (NB) is considered a soft marker for aneuploidy. A soft marker is a sonographic finding that can be associated with, but is not diagnostic of a fetal condition [31]. In the midsagittal plane, the NB is seen as a bright line of greater echogenicity than the skin (Fig. 9.10). The presence of the NB is best assessed at a CRL between 65 and 84 mm correlating to a gestational age of 13–13 + 5 weeks [32]. Criteria for measurement of the nasal bone can be seen in Table 9.7.

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Fig. 9.10

Nasal bone. The hyperechoic nasal tip and skin can be seen with a normal nasal bone noted underneath. The angle of insonation is correct in this image. Note the additional landmark of the rectangular hard palate seen inferior to the nasal bone

Table 9.7

Guidelines for measurement of nasal bone (NB)a

• Measured from 11 to 13 + 6 weeks gestation

• Fetal head, neck and thorax should occupy the entire image

• Measured in the midsagittal view

Echogenic tip of nose should be seen

Third and fourth ventricle seen

Rectangular palate should be seen

• Angle of insonation ~45° to fetal profile

• Brightness of NB equal to or greater than overlying skin

aAdapted from The Fetal Medicine Foundation, www.fetalmedicine.org

A hypoplastic or absent NB has been associated with trisomy 21. One publication reviewed over 35,000 NB examinations from nine different studies and showed that the NB was absent in 65 % of fetuses with trisomy 21 but only in 0.8 % of chromosomally normal fetuses [33]. In the second trimester, this marker becomes less predictive with absent NB seen in 30–40 % of fetuses with trisomy 21 and 0.3–0.7 % of chromosomally normal fetuses [34].

Different methods of reporting observations of the NB yield different results. Some report the NB categorically as “present” or “absent,” while others measure it and report whether it is hypoplastic. Absent NB in the second trimester was seen in 30–40 % of fetuses with trisomy 21 and 0.3–0.7 % of chromosomally normal fetuses. By considering NB hypoplasia or absent nasal bone as a single category, the finding was seen in 50–60 % of fetuses with trisomy 21 and 6–7 % of chromosomally normal fetuses [34]. Other ways to report hypoplasia of the NB include an absolute cutoff of <2.5 mm, gestational age-related cutoff of <2.5th or <5th percentile, a ratio of BPD/NB length or multiples of the median for gestational age with <0.75 MoM being the cutoff for abnormal NB measurement [3536].

It is noteworthy that there is natural variation in the appearance of the NB. Absence of the NB at or before 13 weeks gestation can be a result of delayed ossification instead of absence or hypoplasia [37]. Similarly, ethnic variations exist in the presence and size of the nasal bone. In a study by Cicero et al. the likelihood ratio for trisomy 21 with an absent NB was higher in Caucasian women than in Afro-Caribbean women (likelihood ration of 31 vs. 9) [38]. These variations reinforce that assessment of the NB should not be used in isolation for diagnosis of trisomy 21. On the contrary, it has been used in combination with first-trimester serum screening and NT measurement to increase the detection of trisomy 21–90 % over the 85 % of first trimester combined screening alone [39].

Other Biometric Measurements

Four other biometric measurements are used in the second trimester to estimate gestational age or fetal weight. These measurements include the biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL). The combination of these four measurements for dating and estimation of fetal weight is usually begun starting at 14 weeks gestation, but there is some utility in measuring these parameters in the first trimester.

The measurement of BPD can be useful in the later portions of the first trimester when CRL measurements may be less accurate [18]. This decrease in the accuracy of the CRL may be due to normal changes in embryonic and fetal posture that can distort the CRL measurement. Head circumference can similarly be used in this scenario [40].

A BPD that is inconsistent with expected size may also be an indicator of fetal anomaly. Two studies have reported that small BPD values less than the 5th to 10th percentile may be associated with subsequent diagnosis of open spina bifida [4142].

By 10 weeks gestation, the femur can be identified and measured. Its measurement is usually accurate within 1 week of the fetus’s true gestational age before 20 weeks gestation [40]. This makes it an ideal parameter for quick estimation of gestational age, but care must be taken in order to obtain an accurate measurement. One should be able to see the femoral head or greater trochanter proximally and the femoral condyle distally, and measurement should only include the ossified portion of the bone [43]. This measurement should not be taken in isolation, as it is known that there are some normal variations between ethnic groups. A FL less than the 5th percentile may also be a marker of aneuploidy (such as in trisomy 21) or an early indicator of fetal skeletal dysplasia or early growth restriction [4445].

Summary

Fetal biometry in the first trimester is important because it is our first evaluation regarding the health of a pregnancy. It can give clues regarding risk of fetal anomalies and aneuploidy. Understanding normal and abnormal measurements allows the clinician to accurately evaluate aberrations of early pregnancy and to counsel patients about physiologic and pathologic findings.

Teaching Points

·               The gestational sac is the earliest ultrasound finding of an intrauterine pregnancy.

·               First-trimester ultrasound evaluation is useful in identification of ectopic pregnancies.

·               Failure of appropriate growth of GS and CRL are associated with early pregnancy failure.

·               First-trimester measurement of the CRL is the most accurate ultrasound method of determining gestational age and EDC.

·               Measurement of the nuchal translucency from 10 to 14 weeks gestation in combination with serum markers can be used for risk assessment for fetal aneuploidy.

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American Institute of Ultrasound in Medicine. AIUM practice guideline for the performance of obstetric ultrasound examinations. 2013. http://www.aium.org/resources/guidelines/obstetric.pdf. Accessed 22 Jan 2015.

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Simpson L. First trimester cystic hygroma and increased nuchal translucency. Waltham, MA: UpToDate; 2014 [updated 30 Dec 2014; cited 22 Jan 2015].

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