Medical brain notes 42

  Multifetal Gestation The overall incidence of multiple gestations in the  T  United States is almost 3%, but these pregnanciesaccount for a disproportionate share of perinatal morbidity and mortality. The natural rate of twinning is approximately 1 in 90, and is slightly higher in blacks than in whites.  The rate is rising as a result of an increase in mater-nal age and the more frequent use of assisted reproductive tech-nologies (ART) and ovulation-induction agents.  Since 1980,there has been a 65% increase in the frequency of twins, and a 500% increase in triplet and high-order births. It is estimated that 43% of triplet and high-order gestations result from ART procedures and 38% from ovulation induction; spontaneous conception accounts for the re-mainder. Although the exact mechanism is not known, monozygotic twinning is also higher in pregnancies con- ceived using ART.   Twin gestations can be characterized as dizygotic (fra-ternal) or monozygotic (i...

  NATURAL HISTORY   The following describes the various developmental se-quences possible when the monozygotic conceptus sepa-rates into twins (also called  chorionicity ) [Fig. 17.1]:   ·                Diamnionic/Dichorionic:  If division of the conceptusoccurs within 3 days of fertilization, each fetus will be surrounded by an amnion and chorion.   ·    Diamnionic/Monochorionic:  If division occurs be-tween the 4th and 8th day following fertilization, the chorion has already begun to develop, whereas the amnion has not. Therefore, each fetus will later be sur-rounded by an amnion, but a single chorion will surround both twins.   ·    Monoamnionic/Monochorionic:  In 1% of monozy-gotic gestations, division occurs between days 9 and 12, after development of both the amnion and the chorion, and the twins will share a common sac. Division there-after is in...

  RISKS OF MULTIFETAL GESTATION   Multifetal pregnancies are associated with increased peri-natal morbidity, 3 to 4 times that of a comparable singleton pregnancy. The most significant cause of morbidity is preterm labor and delivery. Compared with singleton pregnancies, which are delivered at an average gestational age of 40 weeks, twins are deliv-ered at an average of 37 weeks, triplets at 33 weeks, and quadruplets at an average of 29 weeks. Thus, with each additional fetus, the length of gestation is decreased by approximately 4 weeks.  Other associated morbidities includeintrauterine growth restriction, hydramnios (in approximately 10% of multiple gestations, predominantly monochorionic gesta tions), preeclampsia (3 times more frequent in twin gestations), congenital anomalies, postpartum hemorrhage, placental abrup-tion, and umbilical cord accidents.  Both spontaneous abor-tions and congenital anomalies are approximately twice as common in multiple gestations (T...

  DIAGNOSIS AND ANTENATAL MANAGEMENT   Most multifetal pregnancies are diagnosed using ultrasound.   On a clinical basis, twin pregnancy should be suspected when the uterine size is large for the calculated gestational age. A difference of 4 cm or more between the weeks of gesta-tion and the measured fundal height should prompt eval-uation with ultrasound to detect the cause (e.g., inaccurate gestational age, multiple gestation, hydramnios, gesta-tional trophoblastic disease, or pelvic tumor).   Serial ultrasound assessments have shown that only 50% of twin pregnancies detected in the first trimester result in the delivery of viable twins. The other 50% of cases deliver a single fetus because of intrauterine demise and ultimate resorption of one embryo/fetus (vanishing twin syndrome). During the first ultrasonographic exam-ination that confirms a twin gestation, chorionicity should be determined because the potential morbidity and mor-tality associated with a monocho...

  INTRAPARTUM MANAGEMENT   Intrapartum management is largely determined by the presenta-tion of the twins.  In general, if the first (presenting) twin isin the cephalic (vertex) presentation, labor is allowed to progress to vaginal delivery, whereas if the presenting twin is in a position other than cephalic, cesarean delivery is often performed. During labor, the heart rate of both fetuses is monitored separately.  Approaches to the delivery of twins vary,depending on gestational age or estimated fetal weight, presenta-tion of the twins, and the experience of the attending physicians.  Regardless of the delivery plan, access to full obstetric, anes-thetic, and pediatric services is mandatory because cesarean delivery may be required on short notice. About 40% of all twin pairs enter labor with both in the cephalic (vertex) pre-sentation. After delivery of the first twin, if the second fetus  remains cephalic, vaginal delivery of the second twin gen-erally ...