Pinky Ronen, M.D.
Identical twins happen when 1 egg is fertilized by 1 then splits (soon after it is fertilized) to form 2 embryos. An “embryo” is the term doctors use to describe when a baby starts to grow. Each embryo grows into a baby. Identical twins have all of the same genes. They are the same sex and look very much alike. Identical twins are less common than fraternal twins.
Fraternal twins happen when 2 different eggs are fertilized by 2 different sperm. Then there are 2 embryos, each of which grows into a baby. Fraternal twins share some, but not all, of the same genes. They are like any 2 siblings, but just share the same birthday. Fraternal twins can be 2 girls, 2 boys, or a boy and a girl.
Certain things increase a woman’s chances of having twins. These include:
These things increase a woman’s chances of having fraternal twins only, not identical twins.
When your doctor or midwife examines you, he or she will feel the size of your uterus. Women who are having twins usually have a uterus that is bigger than expected, given their due date. Also, early in pregnancy, your doctor or midwife will probably do a test called an ultrasound. An ultrasound uses sound waves to create pictures of the inside of your body. This test shows how many babies are in your uterus.
Yes. Women who are pregnant with twins need special prenatal care. They will see their doctor or midwife more often. They will also have frequent ultrasounds to check how their babies are growing. Later on in pregnancy, they might have other tests to check their babies’ health.
Having twins is a bit different than having only 1 baby. Your doctor or midwife will talk with you about how much weight you should gain and how active you should be. He or she will also talk with you about problems that are more likely to happen when a woman is pregnant with twins. The most common problem is that the babies will be born too early, before 37 weeks of pregnancy (3 or more weeks before the due date). Doctors use the term “premature” or “preterm” for babies who are born too early. Being born too early is sometimes a problem. Preterm babies are more likely to be smaller and need to stay in the hospital for longer after birth. They are also more likely to have medical problems, such as breathing problems. Women who are carrying twins are also more likely to get high blood pressure during pregnancy.
It depends on the position of the babies in your uterus, your health, and your babies’ health. If you and your babies are healthy and the first baby is coming head first, you might be able to deliver your babies vaginally. If not, the doctor will do surgery called a c-section to get the babies out.
Twins of the opposite sex are always fraternal. Twins of the same sex can be fraternal or identical. If your twins are the same sex, your doctor or midwife might be able to tell if they are identical by checking your placenta. (The placenta is the organ inside a pregnant woman’s uterus that brings a baby nutrients and oxygen, and carries away waste.) Sometimes, the only way to tell if twins are identical is by doing blood tests.
If you would like to. Most women make enough milk to breastfeed twins. If your babies are born too early, you might need to pump and store your breast milk until your babies are able to drink it. You will probably want to work with a breastfeeding expert, called a “lactation consultant.” That way, you can find a schedule and way of breastfeeding that works best for you and your babies.