A multiple pregnancy is any pregnancy in which two or more babies develop in the uterus at the same time. This includes twins, triplets, and higher-order multiples. About two-thirds of naturally conceived twins are fraternal (from two separate eggs), while the remaining third are identical (from a single egg that splits early in development). Whether twins share a placenta or have their own has a major influence on the risks involved and how the pregnancy is monitored.
How Multiple Pregnancies Happen
There are two fundamentally different ways a multiple pregnancy begins. Fraternal twins form when a woman releases two eggs during ovulation and both are fertilized by different sperm. Each embryo has its own unique genetic makeup, its own placenta, and its own amniotic sac. Fraternal twins can be the same sex or different sexes, and they’re no more genetically similar than any two siblings born years apart.
Identical twins start from a single fertilized egg that splits into two embryos early in development. Because they originate from the same egg and sperm, they share virtually all of their DNA. The timing of that split matters: if it happens within the first few days after fertilization, each twin typically gets its own placenta. If the split occurs three or more days after fertilization, the twins usually share one placenta. In rare cases, a very late split results in twins who share both a placenta and an amniotic sac.
Triplets and other higher-order multiples (three or more babies) can result from multiple eggs being fertilized, a single egg splitting, or a combination of both.
Why Multiple Pregnancies Are More Common Now
Fertility treatments have significantly increased the rate of multiple births. In vitro fertilization (IVF), in particular, historically involved transferring more than one embryo to improve the chances of pregnancy, which raised the odds of twins or triplets substantially. CDC-funded research found that when fertility clinics transferred two embryos instead of one in women under 35, the multiple birth rate was 39.4% compared to just 1.7% with a single embryo transfer.
The shift toward transferring a single embryo has been steadily bringing those numbers down. Studies show a marked, linear reduction in multiple birth rates as clinics adopt single embryo transfer more often, with no significant drop in overall live birth rates. For women under 35, the multiple birth rate fell from 37.5% at clinics rarely using single transfers to 15.2% at clinics doing so at least half the time. Other factors that naturally raise the odds of multiples include older maternal age, a family history of fraternal twins, and certain ovulation-stimulating medications.
How Multiples Are Detected and Classified
Most multiple pregnancies are identified on a first-trimester ultrasound. Beyond simply counting babies, the ultrasound determines two critical details: whether the babies share a placenta (chorionicity) and whether they share an amniotic sac (amnionicity). These distinctions guide the entire plan for prenatal care.
Specific ultrasound signs help make the determination. A thick membrane between the twins with placental tissue wedging into its base, sometimes called the twin-peak sign, indicates two separate placentas. A thin membrane meeting the placenta at a sharp angle, known as the T sign, suggests a shared placenta. When no membrane is visible at all and the umbilical cords appear tangled, the twins share both a placenta and an amniotic sac. The number of yolk sacs visible in early pregnancy also helps confirm how many amniotic sacs are present.
Risks for the Mother
Carrying more than one baby puts greater physical demands on the body. The uterus expands more, blood volume increases further, and the cardiovascular system works harder than in a singleton pregnancy. One of the most significant risks is preeclampsia, a condition involving dangerously high blood pressure. Women with multiple pregnancies face roughly three to four times the risk of developing preeclampsia compared to those carrying one baby. In Scandinavian studies, between 10% and 18% of women with multiples developed the condition.
Preterm labor is the most common complication. The majority of twins are born before 37 weeks, and higher-order multiples arrive even earlier on average. Gestational diabetes, anemia, and excessive nausea are also more frequent. The physical toll of a multiple pregnancy means more frequent prenatal visits and closer monitoring throughout.
Risks for the Babies
The biggest concern for babies in a multiple pregnancy is premature birth and the health problems that come with it, including breathing difficulties, feeding challenges, and longer stays in the neonatal intensive care unit. Growth restriction is also more common because multiple babies compete for nutrients and space.
Twins who share a placenta face an additional, unique risk: twin-to-twin transfusion syndrome (TTTS). This occurs when blood vessel connections within the shared placenta allow blood to flow unevenly, so one twin gets too much blood and the other gets too little. The twin receiving excess blood produces too much fluid, while the other twin produces very little. TTTS affects about 8 to 10% of twins who share a placenta but have separate amniotic sacs. It is diagnosed by ultrasound when one twin’s fluid level is abnormally high and the other’s is abnormally low. Without treatment, TTTS can be life-threatening for both babies, but early detection through regular ultrasound surveillance improves outcomes considerably.
Twins who share both a placenta and an amniotic sac have a somewhat lower rate of TTTS (around 6%) but face the added danger of cord entanglement, since nothing separates them inside the womb.
How Multiple Pregnancies Are Monitored
The level of surveillance depends on what type of multiple pregnancy it is. For uncomplicated twins with separate placentas, the International Society of Ultrasound in Obstetrics and Gynecology recommends a first-trimester scan, a detailed anatomy scan in the second trimester, and growth scans every four weeks after that. Twins who share a placenta are monitored more frequently, typically every two weeks starting in the second trimester, specifically to watch for signs of TTTS or unequal growth. Twins sharing both a placenta and an amniotic sac receive the most intensive monitoring.
Beyond ultrasounds, prenatal visits are generally more frequent in multiple pregnancies. Blood pressure checks, urine testing, and blood work happen on a tighter schedule to catch complications like preeclampsia and anemia early.
Nutrition and Weight Gain
Women carrying twins need more calories than those with a singleton pregnancy, though the exact amount varies based on pre-pregnancy weight and activity level. According to the CDC, a woman who starts a twin pregnancy at a normal weight (BMI of 18.5 to 24.9) should aim to gain between 37 and 54 pounds total. That’s roughly 15 to 25 pounds more than the typical recommendation for a single baby.
The increased demands of growing two or more babies also mean higher needs for iron, folic acid, calcium, and protein. Many providers recommend a prenatal vitamin plus additional iron supplementation, since the expanded blood volume in a multiple pregnancy makes iron-deficiency anemia particularly common.
Delivery Timing and What to Expect
Most twin pregnancies are delivered between 36 and 38 weeks, depending on the type. Twins with separate placentas are often delivered around 37 to 38 weeks, while those sharing a placenta are typically delivered a bit earlier, around 36 to 37 weeks, to reduce the risk of late-pregnancy complications. Twins sharing an amniotic sac are usually delivered by 32 to 34 weeks because of the ongoing risk of cord entanglement.
Whether delivery happens vaginally or by cesarean section depends on factors like the babies’ positions, their health, and whether any complications have developed. When the first twin is positioned head-down, vaginal delivery is often possible. Triplets and higher-order multiples are nearly always delivered by cesarean. Regardless of the method, deliveries of multiples involve larger medical teams, with staff prepared to care for each baby individually.