Incidence
- This is 1–2% of all deliveries.
- The majority (97–98%) are twin pregnancies; 80% of twin pregnancies are dizygous (derived from two separate embryos).
- Multiple pregnancies are becoming increasingly common, primarily as a result of assisted reproductive technology (ART). This is especially true of higher-order multiple pregnancies (triplets and up) which now constitute 0.1–0.3% of all births.
Diagnosis
- Multiple pregnancy should be suspected in women with risk factors (Figure 55.1), excessive symptoms of pregnancy, or uterine size greater than expected.
- Ultrasound will confirm the diagnosis.
Chorionicity (Figure 55.1)
- Chorionicity refers to the arrangement of membranes in multiple pregnancies. It has important prognostic implications.
- Perinatal mortality rate is higher with monozygous (30–50%) than with dizygous twins (10–20%), and is especially high with monochorionic/monoamniotic twins (65–70%).
- Chorionicity is determined most accurately by examination of the membranes after delivery. Antenatal diagnosis is more difficult. Identification of separate sex fetuses or two separate placentas confirm dichorionic/diamniotic placentation.
Complications
Antepartum complications develop in 80% of multiple pregnancies compared with 30% of singleton pregnancies.
1 Multiple pregnancies account for 10% of all perinatal deaths.
2 Preterm delivery increases as fetal number increases: the average length of gestation is 40 weeks in singletons, 37 weeks in twins, 33 weeks in triplets, and 29 weeks in quadruplets.
3 Preterm premature rupture of membranes occurs in 10–20% of multiple pregnancies (see Chapter 59).
4 Fetal growth discordance (defined as a ≥20% difference in estimated fetal weight between fetuses) occurs in 5–15% of twins and 30% of triplets. Perinatal mortality is increased sixfold.
5 Intrauterine demise of one twin (see Chapter 54).
