Reduction of a selected fetus or fetuses in a multichorionic multifetal gestation may be chosen as a therapeutic intervention to enhance survival of the other fetuses. Pregnancy reduction is usually performed between 10 and 13 weeks’ gestation. At this gestational age, most spontaneous abortions have already occurred, the fetuses are large enough for sonographic evaluation, the amount of devitalized tissue remaining after the procedure is small, and the risk of aborting the entire pregnancy as a result of the procedure is low. Typically, potassium chloride is injected into the heart or thorax of each selected fetus under sonographic guidance, taking care not to enter or traverse the sacs of the other fetuses. In most cases, pregnancies are reduced to twins to increase the chances of delivering at least one viable fetus.
Specific risks associated with selective reduction or termination procedures are listed in Table 43-1. Loss rates following selective reduction typically range between 5 and 15 percent, with higher rates in pregnancies that have a higher starting fetal number (see Figure 43-1).
TABLE 43-1. Specific Risks Common to Selective Termination or Reduction
FIGURE 43-1 Histogram showing the rate of pregnancy losses at less than 24 weeks and preterm birth at 25 to 28 weeks as a function of the initial number of multiple fetuses in more than 1000 women who underwent selective reduction of pregnancy from 1995 to 1998. (Modified, with permission, from Elsevier, Evans ML, Berkowitz RL, et al: Improvement in outcomes of multifetal pregnancy reduction with increased experience. Am J Obstet Gynecol 184(2):97–103, 2001.)
Selective termination implies termination of one or more anomalous fetuses, rather than simply reducing the number of fetuses in a higher-order multiple gestation. With the identification of a multiple fetuses discordant for structural or genetic abnormalities, three options are available: (1) abortion of all fetuses, (2) selective termination of the abnormal fetus, or (3) continuation of the pregnancy. Because anomalies are often not discovered until the second trimester, selective termination may be performed later in gestation than selective reduction and may entail greater risk. Usually, the procedure is only performed if the anomaly is severe or if the estimated risk of continuing the pregnancy is greater than the risk of the procedure.
Unless as special procedure as umbilical cord interruption is used, selective termination should be performed only in dichorionic pregnancies, to avoid damaging the surviving fetus. If the pregnancy is dichorionic, injection of potassium chloride can be performed as described earlier. Fetal loss rates at experienced centers approach those for selective reduction.
Women and their spouses who elect to undergo selective termination or reduction find this decision highly stressful. The ethical issues associated with reduction or termination are almost limitless. Prior to pregnancy reduction or termination, the couple should be counseled about risks and benefits, including a discussion of anticipated morbidity and mortality if the pregnancy is continued.
For further reading in Williams Obstetrics, 23rd ed.,
see Chapter 39, “Multifetal Gestation.”