Maternal adaptation in twin pregnancy.

Abstract

T WIN PREGNANCY has intrigued obstetricians for a long time because of the greater incidence of pregnancy complications, such as preeclampsia and preterm delivery, which lead to an increased hazard to both mother and fetus. However, as indicated by survival means in the animal world, in general, the smaller the animal, the greater is the litter size. Also, the combined weight of the offspring or babies is proportional to the size of the mother. Thus, the multiple offspring of a small animal each weigh relatively less than the single offspring of a large mother. A relatively large size at birth would seem to be an evolutionary advantage, and often multiple pregnancy in the human has been considered as an atavistic reversion. Twinning rates in humans are greater when the woman is healthy, tall, well built, and practices good nutrition. This is similar to sheep for example, where the well-nourished ewe produces two or three lambs compared with the poorly nourished ewe, which only produces one. It seems feasible with respect to physiologic terms to consider such women as better reproducers, with an enhanced response in twin pregnancy, ie, the “super mums.“’ As with a singleton pregnancy, the trigger to maternal response is hormonal; there is increased production of both steroid and protein hormones from the fetoplacental unit during twin pregnancy. Like urinary oestriol excretion, which many have shown to be increased in twin pregnancy,2-5 plasma estriol is elevated in twin pregnancies when compared with singleton pregnancies,6 with a similar pattern of change until 36-weeks gestation, when there is a very rapid rise in twin pregnancies to 204% of the mean for singleton births. Likewise, progesterone production is increased from the fetoplacental unit.7,8 Table 1 shows the same response for the