Multiple gestation is a pregnancy with two or more babies developing in the uterus at the same time. The term covers twins, triplets, quadruplets, and beyond. In the United States in 2023, twins occurred at a rate of about 31 per 1,000 live births, while triplets or higher were far rarer at roughly 74 per 100,000.
How Multiple Gestations Happen
A multiple pregnancy starts one of two ways. When the ovaries release more than one egg and each is fertilized separately, the result is fraternal multiples. Each baby has its own placenta and amniotic sac, and they are no more genetically alike than siblings born years apart. About 75% of all twins are fraternal.
The other path is a single fertilized egg splitting into two or more embryos, producing identical multiples who share the same DNA. Among identical twins, roughly three out of four share a single placenta. How early the egg splits determines how much the babies share: they may end up in separate sacs with a shared placenta, or in rarer cases, in the same sac entirely.
Types of Twin Placentation
Doctors classify twins by their placenta and membrane setup, because this directly affects risk levels throughout pregnancy:
- Dichorionic-diamniotic: Each twin has its own placenta and its own amniotic sac. This is the most common arrangement and the lowest risk. All fraternal twins fall into this category, along with some identical twins.
- Monochorionic-diamniotic: The twins share one placenta but have separate amniotic sacs. These are always identical twins. The shared placenta creates specific risks that require closer monitoring.
- Monochorionic-monoamniotic: The twins share both a placenta and a single amniotic sac. This is the rarest and highest-risk configuration, again always identical.
An ultrasound between 11 and 14 weeks is the best time to determine which type of placentation a twin pregnancy has. This scan shapes the entire monitoring plan going forward.
What Increases the Chances
Several factors make a multiple pregnancy more likely. The most significant driver in recent decades has been assisted reproductive technology (ART), including fertility treatments like IVF. Procedures that stimulate ovulation or transfer multiple embryos raise the odds substantially, and the rising use of ART has contributed to an overall increase in multiple births.
Older maternal age also plays a role. As you get older, hormonal changes make it more likely that your body will release more than one egg during a cycle. Higher body weight is another factor linked to spontaneous twin pregnancies. Genetics matter too: if fraternal twins run in your family (specifically on the mother’s side), you’re more likely to release multiple eggs. Race and ethnicity also influence rates, with some populations experiencing higher twin rates than others.
How a Multiple Pregnancy Is Different
Carrying multiples is physically more demanding than a singleton pregnancy and comes with higher rates of several complications. Preeclampsia (dangerously high blood pressure during pregnancy) occurs more frequently, with rates of 2.5% to 6.2% in pregnancies involving twins compared to about 2% in singleton pregnancies. Gestational diabetes is also somewhat more common.
The bigger difference is in delivery timing. A typical singleton pregnancy lasts about 38.6 weeks. Twins arrive at an average of 35 weeks, triplets at around 32 weeks, and quadruplets at roughly 30 weeks. Triplets and higher-order multiples are almost always born preterm. Earlier delivery means babies are more likely to need time in a neonatal intensive care unit as they finish developing outside the womb.
Weight gain recommendations are higher for multiples. For a twin pregnancy, the guidelines call for 37 to 55 pounds (17 to 25 kg) if you started at a normal weight, 31 to 51 pounds (14 to 23 kg) if you were overweight before pregnancy, and 25 to 42 pounds (11 to 19 kg) if you were obese. These ranges support healthy fetal growth and help reduce the risk of very early delivery.
Risks Specific to Shared Placentas
When identical twins share a placenta (monochorionic pregnancies), they face a unique complication called twin-to-twin transfusion syndrome, or TTTS. This happens when the blood vessels connecting the twins within the shared placenta distribute blood flow unevenly. One twin receives too much blood, leading to excess amniotic fluid, while the other gets too little, resulting in very low fluid levels. TTTS typically develops between 16 and 26 weeks of pregnancy.
Doctors diagnose it by measuring the deepest pocket of amniotic fluid around each twin on ultrasound. If the smaller twin’s fluid pocket drops to 2 centimeters or less while the larger twin’s rises above specific thresholds (which vary by gestational age), TTTS is diagnosed. This is one reason monochorionic twins are monitored with ultrasounds far more frequently than dichorionic twins, often every two weeks starting in the second trimester.
What Monitoring Looks Like
If you’re carrying multiples, expect significantly more prenatal visits than in a singleton pregnancy. The early ultrasound between 11 and 14 weeks establishes how many babies there are, whether they share a placenta, and your due date. From there, the monitoring schedule depends on the type of pregnancy.
Dichorionic twins (each with their own placenta) are generally checked with ultrasound every four weeks to track growth. Monochorionic twins need scans roughly every two weeks because of the risk of TTTS and unequal growth. Your provider will also screen more frequently for preeclampsia and gestational diabetes, and you’ll likely discuss the timing and method of delivery earlier than you would with a single baby. Most twin pregnancies are delivered by 37 to 38 weeks even without complications, and many require cesarean delivery, particularly when the first baby is not positioned head-down.
For triplets and higher-order multiples, monitoring intensifies further, with even more frequent ultrasounds and earlier delivery planning. These pregnancies carry the greatest risk of preterm birth and its associated complications, so care teams typically include maternal-fetal medicine specialists from early on.