A multiple gestation pregnancy means carrying two or more babies at the same time. Twins are by far the most common type, occurring at a rate of about 30.7 per 1,000 live births in the United States. Triplets and higher-order multiples are much rarer, at roughly 73.8 per 100,000 births. While many multiple pregnancies result in healthy babies, they carry higher risks for both the mother and the fetuses compared to a single pregnancy, and they require closer monitoring throughout.
How Multiple Pregnancies Happen
There are two basic ways a multiple pregnancy begins. Fraternal multiples (dizygotic) form when the ovaries release more than one egg in a single cycle and each egg is fertilized by a different sperm. These babies have their own placentas and amniotic sacs and are no more genetically alike than any siblings born at different times. Identical multiples (monozygotic) start from a single fertilized egg that splits into two or more embryos. These babies share the same genetic material.
With identical twins, when the embryo splits matters a great deal. An early split, within the first few days after fertilization, gives each baby its own placenta and amniotic sac. A later split, roughly between days four and thirteen, results in the babies sharing a single placenta while usually still having separate amniotic sacs. In very rare cases, splitting after day thirteen can lead to babies sharing both the placenta and the amniotic sac, or even conjoined twins. These distinctions, called chorionicity and amnionicity, are some of the most important details in managing a multiple pregnancy because they determine the level of risk.
What Increases the Chances
The rising rate of multiple pregnancies over the past few decades traces back to two main factors: older maternal age at conception and the increased use of fertility treatments. As women age, hormonal shifts make it more likely for the ovaries to release multiple eggs in one cycle. Fertility treatments amplify this further. Medications that stimulate ovulation can cause several eggs to mature at once, and in vitro fertilization (IVF) historically involved transferring multiple embryos to improve the odds of a successful pregnancy. Even with single embryo transfer during IVF, the twin pregnancy rate is about 1.7%, because a transferred embryo can still split on its own.
Family history and racial background also play a role. Women who are themselves a fraternal twin, or who have fraternal twins in their family, are more likely to release multiple eggs. Fraternal twinning rates vary across populations, with higher rates in some African populations and lower rates in many Asian populations.
How It’s Diagnosed
Ultrasound is the standard way to confirm a multiple pregnancy, and early ultrasound is especially important. Before 10 weeks, the presence of two separate gestational sacs, amniotic sacs, and yolk sacs clearly identifies a pregnancy with two babies that each have their own placenta. Determining whether babies share a placenta is about 99% accurate when assessed before 14 weeks. After 14 weeks, the accuracy drops significantly, to around 77% for identifying a shared placenta. This is why an early first-trimester ultrasound is strongly recommended for any suspected multiple pregnancy.
Some women get an early clue before ultrasound. Higher-than-expected levels of pregnancy hormones can raise suspicion, and symptoms like severe nausea or a uterus that measures larger than expected for the gestational age sometimes prompt an earlier scan. But ultrasound remains the only reliable confirmation.
Why Sharing a Placenta Raises the Stakes
Babies who share a placenta (monochorionic twins) face a fundamentally different biological situation than those who each have their own. A single placenta was not designed to support the growth of two fetuses, and the blood vessel connections within a shared placenta create unique risks.
The most well-known of these is twin-to-twin transfusion syndrome (TTTS), which affects 10 to 15% of twins sharing a placenta. In TTTS, blood flows unevenly between the babies through connected vessels, leaving one baby with too little blood and the other with too much. Without treatment, this can be life-threatening for both. Another complication, selective growth restriction, also affects 10 to 15% of monochorionic twins. This happens when one baby’s portion of the shared placenta is significantly smaller, limiting its growth.
If one baby in a monochorionic pair dies after the first trimester, the surviving twin faces a risk of brain damage in up to 26% of cases, because of the sudden blood pressure changes through those shared connections. Monochorionic twins also tend to have lower birth weights, are born earlier, and experience higher rates of complications compared to twins with separate placentas.
Risks for the Mother
Carrying more than one baby places greater physical demands on the body. Preeclampsia, a dangerous condition involving high blood pressure and organ stress, occurs more frequently in multiple pregnancies. Gestational diabetes, anemia, and excessive nausea are also more common. The uterus stretches more, which increases the risk of preterm labor, and the larger placental area raises the chance of placental problems during delivery.
Postpartum hemorrhage, or heavy bleeding after birth, is more likely with multiples because the overdistended uterus may not contract as efficiently afterward. Cesarean delivery rates are also higher, particularly for monochorionic twins and any pregnancy with three or more babies.
Prenatal Monitoring
Multiple pregnancies require more frequent checkups than singleton pregnancies. For twins sharing a placenta, ultrasound monitoring for TTTS and uneven growth should ideally start by 16 weeks and continue every two weeks until delivery. Twins with separate placentas still need more frequent scans than a single pregnancy, typically every four weeks or so to track growth differences between the babies.
A fetal anatomy scan is recommended at 18 to 20 weeks, just as with a single pregnancy, to check for structural abnormalities. For pregnancies with three or more babies, or for the rare case of twins sharing a single amniotic sac, monitoring is highly individualized and typically involves a maternal-fetal medicine specialist.
Weight Gain and Nutrition
The body needs significantly more calories and nutrients to support multiple babies. Weight gain recommendations for a twin pregnancy are higher than for a singleton. For women starting at a normal weight, the recommended gain is 37 to 54 pounds. For overweight women, the target range is 31 to 50 pounds, and for obese women, 25 to 42 pounds. For triplets and beyond, there isn’t enough data to set firm guidelines, so recommendations are made on an individual basis.
Protein, iron, calcium, and folic acid all become more important with multiples. Many women carrying twins or more develop anemia because the increased blood volume and the demands of multiple placentas deplete iron stores faster. Adequate nutrition in the second and third trimesters plays a meaningful role in fetal growth and in reducing the severity of growth differences between the babies.
Delivery Timing and What to Expect
Most multiple pregnancies deliver earlier than the standard 40-week mark for singletons. Twins sharing a placenta are generally delivered earlier than those with separate placentas because of the higher complication risk. The goal is to balance the risks of prematurity against the risks of continuing the pregnancy.
Many twin pregnancies can be delivered vaginally, particularly when the first baby is positioned head-down. However, cesarean delivery is more common with multiples and is often recommended for monochorionic twins sharing an amniotic sac, for triplets and beyond, or when the babies are in unfavorable positions. Regardless of the delivery method, most providers recommend delivering at a facility with a neonatal intensive care unit available, since premature birth remains the most common complication of multiple gestations.
Babies born from multiple pregnancies are more likely to need time in the NICU, even when born close to full term. Breathing support, temperature regulation, and feeding assistance are the most common reasons. The earlier the babies arrive, the longer the NICU stay tends to be, with babies born before 32 weeks typically needing several weeks of specialized care.