In the United States, 1 in 35 are twin births. 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. Multiples are more common in women:
Fraternal twins occur when two eggs are fertilized (the two eggs can come from the same or different ovaries), producing two embryos of the same or different sex (50% of the time the twins will be of different sex). These twins are genetically no more similar than other siblings. Each fraternal twin has its own placenta, although the placentas may fuse or merge and appear as one organ.
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 (although not all same sex twins are identical) and may have separate placentas or share one. Although a shared single placenta always indicates identical twins, the presence of two placentas does not indicate if twins are identical or fraternal.
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.
A multiple gestation is 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 cesarean section.
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 also 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 there is concern for their well-being. 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.
Compared with a singleton pregnancy, a multiple pregnancy is different in several ways. Here's what you may experience: