Multiple gestation is a pregnancy with two or more babies developing in the uterus at the same time. This includes twins, triplets, and any higher number. About 31 out of every 1,000 births in the United States are twins, while triplets or more occur in roughly 74 out of every 100,000 births. These pregnancies carry higher risks for both mother and babies compared to a single-baby pregnancy, and they require more frequent monitoring throughout.
How Multiple Pregnancies Form
There are two fundamental ways a multiple pregnancy begins. Dizygotic (fraternal) twins come from two separate eggs fertilized by two separate sperm. Each baby has its own placenta and amniotic sac, and they share roughly 50% of their DNA, just like any siblings. Fraternal multiples can be any combination of sexes.
Monozygotic (identical) twins come from a single fertilized egg that splits into two embryos. The timing of that split determines how the babies share their support structures. If the split happens within the first three days, each twin gets its own placenta and amniotic sac. If it happens between days four and eight, the twins share a placenta but have separate amniotic sacs. A later split can result in twins sharing both a placenta and a sac, which is the rarest and highest-risk arrangement. Among identical twins born alive, about 70 to 75% share a single placenta.
Why Multiple Pregnancies Are Increasing
Two major factors have driven a significant rise in multiples over the past 50 years: delayed childbearing and assisted reproductive technology (ART). Women who conceive later in life are more likely to release multiple eggs during ovulation. Fertility treatments, including ovulation-stimulating medications and in vitro fertilization, dramatically increase the odds. Births from ART have gone from representing about 2% of births to 30 to 35% of multiple gestations. Family history also plays a role, particularly for fraternal twins, since the tendency to release more than one egg per cycle can be inherited.
Vanishing Twin Syndrome
Not every multiple pregnancy that’s detected early results in multiple babies. Vanishing twin syndrome occurs when one embryo in a multiple pregnancy spontaneously dies and is partially or fully reabsorbed by the body, typically in the first trimester. This happens in an estimated 15 to 36% of twin pregnancies and 30 to 50% of pregnancies that start with three or more gestational sacs. In most cases, the surviving baby is unaffected and the pregnancy continues normally. Many people never know it happened unless an early ultrasound captured the additional sac before resorption.
Risks to the Mother
Carrying more than one baby puts greater physical demands on the body and raises the likelihood of several complications. Preeclampsia, a dangerous condition involving high blood pressure and organ stress, is three to four times more common in multiple pregnancies than in singleton pregnancies. In Scandinavian studies, preeclampsia affected 10 to 18% of women carrying multiples. The risk of gestational diabetes, anemia, and excessive bleeding after delivery is also elevated. Preterm labor is one of the most common complications, since the uterus is stretched beyond what a single pregnancy requires.
Risks to the Babies
More than half of multiple birth babies are born with low birthweight, compared to an average of about 7.6 pounds for a single newborn. Preterm birth is the primary reason. Twins arrive earlier on average, and triplets earlier still. Nearly all low birthweight babies require care in a neonatal intensive care unit (NICU) until they gain enough weight and stability to go home.
Babies who share a placenta face additional, unique risks. Twin-to-twin transfusion syndrome (TTTS) occurs when blood flow between the twins becomes unbalanced through shared blood vessels in the placenta, leaving one twin with too much blood and the other with too little. This is why shared-placenta pregnancies are monitored so much more closely.
Prenatal Monitoring
The type of multiple pregnancy determines how often you’ll be seen for ultrasounds. The key distinction is whether the babies share a placenta or each have their own, and this is established at the first-trimester scan.
If each twin has its own placenta (dichorionic), the standard recommendation is a first-trimester scan, an anatomy scan around 20 weeks, and then ultrasounds every four weeks after that. Starting at 24 weeks, blood flow in the umbilical arteries is checked at each visit.
If the twins share a placenta (monochorionic), monitoring is twice as frequent. After a first-trimester scan, ultrasounds should happen every two weeks starting at 16 weeks. This schedule exists specifically to catch TTTS early, since timely detection significantly improves outcomes. At each scan, the care team measures each baby’s growth, checks the amniotic fluid around each twin, and evaluates blood flow patterns. Any weight difference between the babies is tracked carefully from 20 weeks onward.
For complicated pregnancies of either type, scans happen even more often depending on the specific issue.
Weight Gain Guidelines
Gaining the right amount of weight in a twin pregnancy supports healthy birth weights and reduces the chance of preterm delivery. The Institute of Medicine recommendations are based on your pre-pregnancy BMI:
- Normal weight (BMI under 25): total gain of 37 to 54 pounds, or about 1 to 1.5 pounds per week
- Overweight (BMI 25 to 30): total gain of 31 to 50 pounds, or about 0.8 to 1.4 pounds per week
- Obese (BMI over 30): total gain of 25 to 42 pounds, or about 0.7 to 1.1 pounds per week
These targets are notably higher than for a singleton pregnancy. Meeting them typically requires eating more calories and paying close attention to protein, iron, calcium, and folate intake, since your body is supporting the growth of two or more babies simultaneously.
When and How Delivery Happens
The ideal delivery timing for multiples depends on the type of pregnancy and whether complications have developed. For twins with separate placentas and no complications, delivery is typically planned at 38 weeks. Twins sharing a placenta but with separate sacs are usually delivered between 34 and 37 weeks. Twins sharing both a placenta and an amniotic sac carry the highest cord-related risks and are delivered between 32 and 34 weeks.
If one twin is growing significantly slower than the other, delivery moves earlier. For separate-placenta twins with growth restriction, the window shifts to 36 to 37 weeks. For shared-placenta twins with growth restriction, it moves to 32 to 34 weeks. When a mother develops preeclampsia or other complications, the timeline is adjusted accordingly.
Vaginal delivery is possible for twins in many cases, particularly when the first baby is positioned head-down. Triplets and higher-order multiples are delivered by cesarean section due to the increased risk of umbilical cord complications during delivery. For any delivery planned before 34 weeks, the mother receives steroid injections beforehand to help the babies’ lungs mature more quickly.