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:
- Older than 35
- Who have had previous multiple births
- With a family history of multiple births
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:
- Pregnancy symptoms, such as morning sickness and breast enlargement, may be more severe because of higher levels of pregnancy hormones.
- Weight gain will be substantial. You’ll be advised to gain between 35 and 45 pounds if your average weight was in the normal range at the beginning of your pregnancy. Optimal weight gain should be discussed with your obstetrical provider. In general, you should add 300 calories per day to your diet for each baby, but consultation with a nutritionist and your obstetrical provider will help guide you with your optimal diet composition and caloric intake. An average weight gain of 1.5 pounds per week during the second and third trimester is generally recommended.
- Symptoms of later pregnancy may occur earlier and be more severe than in singleton pregnancy. A very heavy, stretched uterus will place pressure on bodily organs, causing easy fatigue, shortness of breath, heartburn, constipation, pelvic discomfort, urinary leakage, back pain and hemorrhoids.
- Complications such as preterm labor, preterm delivery, premature rupture of the amniotic membranes, preeclampsia, placenta previa, prolonged labor and newborn problems are more common in a multiple pregnancy. There is an increased risk of perinatal loss and poor perinatal outcome (mortality and morbidity) with multiple gestations. Of particular concern is a higher risk of premature delivery from either preterm labor or preterm, premature rupture of memberanes. While many twins deliver at 37 to 38 weeks of gestation, the higher than average risk for premature delivery means you should immediately contact your provider if you have regular contractions, bleeding or leaking fluid.
- Fetal growth is frequently monitored throughout the pregnancy by ultrasound, both for adequacy or interval growth and with respect to each other. Multiple gestations ideally should display similar growth and weight gain. If the weights differ by greater than 20%, then there is “discordant growth” and delivery may be necessary; if the weights are similar but both are under the 10th percentile for gestational age, then delivery may be necessary. Discussion with the obstetrical provider is critical in either situation.
- There is a higher percentage of low birth weight (birth weight less than 5.5 pounds).
- Identical twins that share a single placenta are at risk for twin to twin transfusion syndrome in which blood from one is transferred to the circulation of the other. This can compromise the health of each twin. It is something that can be detected by ultrasound.