MULTIPLE
GESTATION
In roughly 3.4% of births in the United States, two or more fetuses
coexist. This rate is rising, having done so by about 70% since 1980. In
approximately 1/10,000 births, there will be spontaneously occurring triplets.
The rise in multiple births is thought to be due to the use of fertility drugs
and other technologies, and to an increased rate of childbearing in women older
than 30, who are more likely to conceive multiples. Interestingly, up to 50% of
twin pregnancies identified in the early weeks will silently abort one fetus
(with or without bleeding).
The first weeks following fertilization represent the most critical period for the success of a pregnancy. It is also during this period that the fertilized egg can split to form one or more “identical” (monozygotic) embryos. This occurs in about 4/1000 births. When more than one egg is released and fertilized during the same menstrual cycle (naturally or through assisted ovulation), fraternal (dizygotic) twins, triplets, or higher order multiples can result. Dizygotic twins are more common in mothers who are themselves a dizygotic twin.
Multiple gestations are
responsible for a disproportionate share of perinatal morbidity and mortality,
accounting for 17% of all preterm births (before 37 weeks of gestation), 23% of
early preterm births (before 32 weeks of gestation), 24% of low-birth-weight
infants (2500 g), and 26% of very-low-birth-weight infants (1500 g). Hospital
costs for women with multiple gestations are on average 40% higher than for
women with gestational-age-matched singleton pregnancies because of their
longer length of stay and increased rate of obstetric complications.
The possibility of a
multiple gestation should be considered any time there is a discrepancy between
uterine size (larger than expected) and gestational age or when multiple fetal
heart tones are heard by auscultation or Doppler ultrasound study. Establishing
the presence and number of fetuses early in gestation is important not only for
family reasons, but also the presence of a multiple gestation increases the risk
of gestational diabetes and other abnormalities. Multiple gestations also cause
different levels of gestation-sensitive laboratory tests, such as maternal
serum-fetoprotein, which would be interpreted as abnormal in a normal,
singleton pregnancy. Genetic amniocentesis may be considered for selected
patients because twin gestations have twice the rate of abnormalities
(monozygotic have a 2% to 10% rate). Furthermore, in multiple gestations,
chorionicity can best be determined ultrasonically early in pregnancy by
assessing the thickness of the dividing membrane between the gestational sacs;
as pregnancy progresses, this distinction becomes more difficult.
To provide nutritional
support for a multiple gestation, the mother should increase her caloric intake
by roughly 330 kcal (twins) more than that normal for pregnancy. Appropriate
iron and folic acid supplementation should also be ensured.
Perinatal morbidity and
mortality for multiple gestations is 2 to 5 times higher than for singleton
gestations. Preterm delivery (50%) is the most common cause of morbidity or mortality. Indeed, most
twin pregnancies will deliver between 36 and 38 weeks of gestation. Other
complications include intrauterine growth restriction (12% to 47% vs. 5% to 7%
in singletons) or discordant growth, cord accidents, hydramnios, con- genital
anomalies (2 times increase), and malpresentation. Monozygotic twins have a 1%
incidence of a monoamniotic sac that carries a 50% fetal mortality due to cord
entanglement or conjoined twins. One-fifth of triplet pregnancies and one-half
of quadruplet pregnancies result
in at least 1 child with a major long-term
handicap, such as cerebral palsy. When matched for gestational age at delivery,
infants from multifetal pregnancies have a nearly threefold greater risk of
cerebral palsy.
Maternal complications of multiple gestation include abruptio placentae, placenta previa, preeclampsia, anemia, hyperemesis gravidarum, pyelonephritis, cholestasis, postpartu hemorrhage, and an increased operative delivery rate.
