In the United States, the overall odds of having twins are 1 in 35. The multiple pregnancy rate is higher in patients who have used fertility drugs and/or have had in vitro fertilization. The incidence of twin births has increased during the past 15 years because of the growing use of assisted reproductive techniques. Twins can be fraternal or identical.
Fraternal twins occur when two eggs from one or both ovaries are fertilized, producing two embryos of the same or different sex. These twins are no more similar than other siblings. Each twin generally has its own placenta, although the placentas may fuse or merge and appear as one organ. The number of placentas does not always indicate if twins are fraternal or identical. Identical twins develop from a single egg that cleaves after fertilization. The cleaved egg separates, creating two embryos with the same genetic material, the same genes. Identical twins are always the same sex and frequently have one placenta.
In the United States, triplets and higher order multiple births (quadruplets, quintuplets, sextuplets and septuplets) occur in 1 in 660 pregnancies. The use of fertility drugs to stimulate the release of more than 1 egg is the most significant risk factor for multiple births. Multiples are more common in women older than age 35; in women with a family history of multiple births, or who have had previous multiple births.
A multiple gestation is often diagnosed by ultrasound examination. Ultrasound is then used to monitor fetal growth and development as the pregnancy advances. The use of ultrasound also provides valuable information about the position of the fetuses, especially at the time of delivery. This information helps determine the best or safest route of delivery, vaginal or by Caesarean section. Ultrasound is also used to monitor the dimensions and shape of the woman's cervix and may help identify premature cervical dilation and possibility of preterm delivery. More research is needed regarding the value of ultrasound prediction of premature cervical dilation and risk of preterm delivery.
Compared with a singleton pregnancy, a multiple pregnancy is different in several ways. Here's what you may experience:
The number of fetuses, their gestational age and their position will greatly influence what manner of delivery is chosen. Vaginal delivery is possible for a "mature" twin gestation at term when both twins are in the head down, "vertex", position. If the first, or presenting twin, is vertex and the second is breech, it is possible to proceed with a vaginal birth. There is greater potential need for cesarean delivery of the second twin. Sometimes a cesarean section is advisable for both babies, especially if the twins are premature, small or are in distress. If the presenting twin is breech, most providers will advise a cesarean birth. These matters should depend on the individual woman's circumstances, in consultation with her obstetrical provider. In general, higher order multiples such as triplets or quadruplets, are delivered by cesarean section.
Last updated July 01, 2009