All About Twins, Triplets, and More

If two or more babies grow in a woman’s uterus at the same time, they’re known as “multiples.” Sometimes they look exactly alike -- identical -- and sometimes they don’t look any more alike than typical siblings -- fraternal. They happen in different ways.

A fertilized egg is made with one egg and one sperm. If it splits in two, which happens sometimes, you have identical twins. If one of those eggs separates again, you’ll have identical triplets, and so on. All the babies start with the same gene set: They’re either all boys or all girls, and they’ll look alike. Identical multiples happen in three or four of every 1,000 live births.

Sometimes, more than one egg comes out of a woman’s ovary in a single month. If each one is fertilized by different sperm, fraternal multiples happen. Unlike identicals, the genes of fraternals are as different as any other sibling with the same parents. These kind of multiples are born more often than identicals.

If a woman has two or more eggs during their fertility window, it’s possible that each one could be fertilized at different times -- even by different men. So it’s possible for multiples to be born with different fathers.

Parents of twins often say their children have a special language they use only with each other. Apparently, the communication starts early. One study found that by the 14th week of pregnancy, twins make intentional movements toward each other. More research is needed to see if this holds true for other multiples -- triplets, quadruplets, etc.

A cesarean section, also called a C-section, is a way of giving birth through a cut in the abdomen. It’s usually done to protect the health of the mother and their babies. Women are more likely to have one if they have twins, and most triplets and higher-number multiples are born this way. 

Why are some women more likely to have multiples? A few things can affect the odds. For example, if a woman hasn't been able to get pregnant, their doctor may recommend medication to make their ovaries release more eggs. This can boost the chances of them getting pregnant -- and the chances of them having fraternal multiples .

This is when a doctor takes eggs from a woman’s ovaries, usually after they've taken fertility drugs. The eggs are fertilized with sperm outside the womb, and then put back inside their uterus. Because this can be tricky and unpredictable, two or more embryos are often put back to make it more likely that at least one will grow and develop. Sometimes two or more do, and the parents have fraternal multiples.

More than 35% of American women who have babies are over 30, and that’s thanks in part to fertility treatment. Even without a doctor’s help, women over 30 are more likely to release two or more eggs at once, possibly because their bodies make more of an ovary-stimulating hormone to give their ovaries a boost. And African-American women are most likely to have twins, while Asian women are least likely.

Moms who have multiples are an average of 1 inch taller than other moms. A hormone that taller women have more of -- insulin-like growth factor, or IGF -- may be the reason. It may be that IGF makes a woman’s ovaries release more eggs, but more research is needed to know for sure. 

One study found that women who drank more milk or milk products had twins more often. Some scientists think these lead your body to make more IGF, which can lead to more eggs in a monthly cycle.

A woman with a mother or sister who has fraternal twins is about twice as likely to have them as well. And women who have a higher body mass index (BMI) also have a higher rate of fraternal multiples. BMI is a measure of your body fat -- higher-than-normal BMI is generally bad for your health.

This is the most common complication with multiples. A “full-term” baby is delivered around 39 or 40 weeks, but most multiples are born “preterm,” or under 37 weeks. They’re 6 times more likely to be preterm than a single baby. Babies born before 32 weeks are more likely to have long-term health issues like hearing loss, vision problems, and possibly brain damage.

This causes high blood pressure and other problems. It can happen in any pregnancy, but it’s more common with multiples. A rise in blood pressure is usually the first sign, but women who have it also may have headaches, vision problems, nausea, and vomiting. It can be dangerous to the mother and babies, but there’s medication to lower blood pressure and manage other symptoms. The condition goes away after the mother gives birth.

Because multiples have a higher chance of problems and earlier deliveries than single babies, doctors like to keep a closer eye on them. They track the babies' growth and development, monitor the mother’s health, and watch for signs of early labor. They also might do ultrasounds -- a way of looking at the babies in the womb -- and other tests to make sure everything’s going OK.