Multiple Pregnancy Health Article

Definition

A multiple pregnancy is a pregnancy in which more than one fetus develops in the uterus at the same time. Multiple pregnancies occur in 1–2% of pregnancies. The rate of twinning (the bearing of twins) is believed to be underestimated, as twin pregnancies with a singleton (an offspring born singly) birth are usually not recorded as twins.

Description

A multiple pregnancy may be the result of the natural process of twinning, or it may be the result of the woman having taken fertility drugs. Because of the increase in artificial reproductive technology (ART), the incidence of multiple pregnancies has increased. An April 1999 National Vital Statistics report from the Centers for Disease Control and Prevention (CDC) states that since 1980 the number of twins has risen by 52% and the number of triplets and high order multiples (more than three) has increased by 404%. An older maternal age and the use of fertility techniques are seen as the two major factors in these increases. While singletons have a 10% risk of being born preterm, multiple births have a 57% chance of being born prematurely. Premature birth places a neonate at higher risk for morbidity and mortality.

There are two categories of twins: monozygotic and dizygotic. Monozygotic twins are twins that have developed from a single fertilized ovum that split during embryonic development. These twins have the same genetic makeup and are always the same sex. They may be surrounded by one chorion (the outer embryonic membrane of the developing fetus), or may each have their own chorion. They may be surrounded by one amniotic sac (innermost of the membranes surrounding the embryo) or may each have their own amniotic sac. They may share a placenta or may each have their own placenta. These different possibilities depend on the time of the embryonic development at which the division took place. About two to 5% of monozygotic twins will share one amniotic sac. This rare occurrence puts the twins at risk for umbilical cord entanglement, cessation of blood flow, and death.

Double survival of monoamniotic twins is rare. Monozygotic twins may be referred to as identical. Dizygotic twins have developed from two fertilized ova. Their genetic makeup is different, and they are no more similar as any two siblings in a family. They may be the same or different sex. Each have their own chorion, amniotic sac, and placenta. While each twin has its own placenta, the placental implantations may be close enough that they fuse into one. Dizygotic twins may be referred to as fraternal. Multiple pregnancies of three or more fetuses may be the result of a single fertilized egg that splits, of multiple egg fertilizations, or a combination of the two processes.

Twins may not grow at the same rate. When there is 25% or more disparity between them, this is referred to as discordance, which occurs in about 10% of twin pregnancies. An extreme case of discordance occurs in the condition called twin-to-twin transfusion, also known as twin oligohydramnios polyhydramnios sequence. In this situation, one twin becomes the donor twin (receives too little blood from vessels in the fetuses' shared placenta that connect their blood circulations) and the other twin is the recipient (receives too much blood). The donor twin becomes small, pale, hypotensive, and anemic, with very little amniotic fluid. The recipient twin is large, polycythemic, hypertensive, with an excess of amniotic fluid. Both are at risk for heart failure and death.

At the time of delivery twins may be in any of the following combinations: vertex-vertex, breech-vertex, vertex-breech, breech-breech, vertex-transverse, or breech-transverse.