In the past 40 years, the rate of twin birth has nearly doubled. This trend is primarily attributable to two factors: women delaying conception to a later age when twin pregnancies are more common, and increased use of assisted reproductive technology by infertile couples.
Nearly every possible obstetrical complication occurs more commonly if a woman carries twins. Hypertensive disorders, including pre-eclampsia, occur 2.5 times more often. The rates of excessive morning sickness, gestational diabetes, anemia, cesarean delivery and post-partum depression are also elevated.
Perhaps the greatest risks for a woman pregnant with twins are borne by the twins themselves. There is a fivefold increased risk of stillbirth and a sevenfold increased risk of neonatal death. The risks are primarily explained by the increased incidence of premature birth. The increase in the rate of early delivery is related to two factors: higher rates of spontaneous preterm labor and early delivery due to a maternal obstetrical high-risk problem.
Fraternal twins develop from separate eggs. They must have separate placentas and always develop in separate sacs. Identical twins develop from a single egg. They can exist in one of three scenarios: separate placentas and separate sacs; a shared placenta and separate sacs; shared placenta and shared sac.
Invariably, the first question always asked of a woman pregnant with twins is "Are they identical?" Unless the twins are separate genders, before birth the answer is not straightforward. Twins that develop in separate sacs are not always fraternal. The best way to determine if the twins are identical will be to find out how many placentas are present.
If the twins share a placenta, they must be identical. If they do not share a placenta, they may be either fraternal or identical. If the placentas are not shared, the only way to determine if twins are not identical is the presence of different genders.
Perhaps the best way to predict risk for a twin pregnancy is to determine if the twins share a placenta. This is because the presence of a single, shared placenta increases the chances for neonatal death, premature birth, birth defects and poor in-utero growth.
Early ultrasound is the only reliable way to determine if the twins share a placenta. This is one of the many benefits of a routine, first trimester ultrasound.