How Twins Occur

When a sperm meets an egg in the fallopian tube, fertilization—union of egg and sperm—can occur. If this happens, the fertilized egg moves down the fallopian tube to the uterus where it attaches and grows into a fetus. During this process, twins can form.

Twins can be either fraternal or identical. Most are fraternal twins—each develops from a separate egg and sperm. Fraternal twins each have their own placenta and amniotic sac. Because each twin develops from the union of a different egg and a different sperm, these twins may not look alike. The twins can be boys, girls, or one of each.

Fraternal twins come from two eggs and have separate placentas (left). Identical twins come from one egg and may share the same placenta (right).

Identical twins are more rare. They occur when a fertilized egg splits early in pregnancy and develops into two fetuses. Identical twins may share a placenta, but each baby usually has its own amniotic sac. Identical twins are the same sex and have the same blood type, hair color, and eye color, and they look very much alike.

Some families are more likely than others to have fraternal twins. Women who take fertility drugs or have in vitro fertilization also have a greater chance of having twins.

Diagnosis

Most twins are diagnosed before delivery. Your doctor may suspect that you are carrying twins if your uterus grows more quickly or is larger than expected. However, twins usually are diagnosed by using an ultrasound exam. Ultrasound also may be able to tell if the twins are identical or fraternal.

Prenatal Care

You will need special prenatal care with twins. You should see your doctor more often and you may need special tests. Plan to take childbirth classes during your 4th to 6th month of pregnancy. Ask your doctor about classes for parents expecting twins.

Nutrition

When pregnant with twins, you will need to eat more than if you were carrying one baby. Eating well and gaining weight are important for your health and the health of your twins. Your body must nourish the babies and meet your energy needs as well. You will need to eat about 2,700 calories every day, if you are of average weight.

You should gain more weight when carrying twins than if you were having only one baby. Plan to gain 35–45 pounds. In the first half of pregnancy, you should gain about 1 pound per week. In the second half, you should aim for a little more than 1 pound each week. If you are underweight, you may need to gain more. If you are overweight, you may need to gain less.

In addition to a prenatal vitamin, your doctor may prescribe extra vitamins and minerals to help your babies grow. Anemia (iron-poor blood) is more common in women who are pregnant with twins, so it's especially important to take your prenatal vitamins and iron as prescribed. Folic acid also is important for twins. It is hard to get all the folic acid you need just from your diet, so you should take a prenatal vitamin that contains folic acid.

Rest

Twins can make you more uncomfortable than usual during pregnancy because the uterus becomes much larger. Resting during the day will help give you energy. Talk with your doctor about your job or your work at home. Some women may be asked to restrict their activity.

Complications

The risk of certain complications is higher in a twin pregnancy. The mother is more likely to develop high blood pressure or anemia, and the babies are more likely to be born small. The mother also is at increased risk for preterm labor. Sometimes these problems can be prevented with early detection and care.

Preterm Labor

Preterm labor is labor that starts before the end of 37 weeks of pregnancy. This can result in preterm birth, the most common problem of multiple pregnancy. About half of all twins are born preterm. When babies are preterm, they often have problems breathing and eating. They will have to stay in the hospital nursery longer than usual. Extremely preterm babies can die or have severe mental or physical problems, even with the best of care. In others, problems can occur as they grow and develop.

If you have preterm labor, you may be given injections of a steroid medication. This can help the babies' lungs work better. If preterm labor is found early enough, delivery can sometimes be postponed for several days. This allows time for the steroid medication to help treat the babies' lungs. Postpon-ing the birth for even a few more days can make a big difference.

Preeclampsia

Preeclampsia is a condition of pregnancy in which there is high blood pressure and protein in the urine. Preeclampsia can be mild or severe.

This condition is harmful because it can reduce the blood flow through the vessels in the uterus. This may deprive the babies of oxygen and nutrients. As a result, the fetuses may not grow as much as they should. It also can damage the woman's heart, liver, kidneys, and brain. In severe cases, preeclampsia may cause seizures and threaten your life. The babies may need to be delivered early if your blood pressure becomes too high.

Growth Problems

Twins are more likely to be smaller than average. Ultrasound is often used to check the growth and the amount of amniotic fluid of each baby.

Twins are called discordant if one is much smaller than the other. Discordant twins are more likely to have problems during pregnancy and after birth. Twins may be discordant because of poor functioning of the placenta or twin-twin transfusion syndrome (TTS).

TTS can develop when identical twins share a placenta. The blood passes from one twin to the other through their shared placenta. The twin that gives the blood will be very small and have too little amniotic fluid. The other twin can have too much blood and amniotic fluid. Some of the extra fluid may need to be removed. If TTS is severe, the twins may have to be delivered early or, in rare cases, they may die.

Fetal Loss

In some twin pregnancies, one of the babies dies. If this happens in early pregnancy, you may have some spotting or bleeding from your vagina. This does not harm you or the other baby.

Later in pregnancy, death of one of the babies is more serious, but often you can still have one healthy child. Losing a baby when you are still pregnant with another can be very hard for you and your family. It can help to talk with your doctor, nurse, or a counselor about your feelings.

Monitoring A Twin Pregnancy

Many techniques are used to check the well-being of your babies. Your doctor may use some or all of these techniques to monitor your babies:

• Have you count your babies' movements, called kick counts, at home
• Examine your cervix by physical exam or ultrasound exam for changes that may show early signs of preterm labor
• Repeat ultrasound exams to check the babies' growth
• Measure the babies' heart rate in response to their own movements (nonstress test)
• Perform a biophysical profile, which includes checking the babies' heart rate, body movement, muscle tone, and the amount of amni-otic fluid by ultrasound

In some cases, twins can be delivered by vaginal birth. In others, a cesarean birth, in which the baby is delivered through a cut made in your abdomen and uterus, may be needed. How your babies are born depends on certain factors, including:

• Position of each baby
• Weight of each baby
• Your health
• Health of the babies

Twins usually can be born vaginally if they both are in the head-down position. A vaginal birth also may be possible when the lower twin is in the head-down position but the higher twin is not. Once the first twin is born, the other twin can sometimes be turned or delivered with feet or buttocks first. When this can't be done, the second twin is delivered by cesarean birth. When the lower twin is not in the head-down position, both twins are delivered by cesarean birth.

Caring for Your Newborn Twins

Most twins do well at birth and can be cared for like any other healthy babies. However, caring for twins is stressful at times. Get as much rest as you can. You may feel overwhelmed, or even sad sometimes. If this occurs, let you doctor know.

If the babies are born early, they may need special medical care to breathe, eat, and keep warm. Preterm and small twins may be cared for in a special nursery called a neonatal intensive care unit at the hospital.

Many women wonder if they can breastfeed more than one baby. The answer is yes. Mother's milk is the best food for any infant. It has the right amount of all the nutrients the baby needs. When you breastfeed, your milk supply will increase to meet the amount needed by your babies. You will need to eat healthy foods and drink plenty of liquids. If your babies are premature, you can pump and store your milk until they are strong enough to nurse from the breast. You may find it helpful to talk with a lactation specialist who is trained to teach women about breastfeeding.

Finally...

Having twins can be an exciting time. You can give your babies the best possible start by taking good care of yourself. Learn the warning signs of problems, and talk to your doctor or nurse about your questions and concerns.

It also is helpful to talk with others who have twins. There are organizations and support groups that provide services for parents of twins. Ask your doctor or nurse to recommend one.

Glossary

Amniotic Sac: A fluid-filled sac in the mother's uterus where the fetus develops.

Cesarean Birth: A surgical procedure in which the babies are delivered through an incision in the mother's abdomen and uterus.

Discordant: A large difference in the size of fetuses in a multiple pregnancy.

Fraternal Twins: Twins that have developed from more than one fertilized egg; they are not genetically identical and each has its own placenta and amniotic sac.

Identical Twins: Twins that have developed from a single fertilized egg; they are usually genetically identical and may or may not share the same placenta and amniotic sac.

In Vitro Fertilization: A procedure in which an egg is removed from a woman's ovary, fertilized in a dish in a laboratory with the man's sperm, and then reintroduced into the woman's uterus to achieve a pregnancy.

Placenta: Tissue that provides nourishment to and takes away waste from the fetus.

Twin-Twin Transfusion Syndrome (TTS): A condition of identical twin fetuses when the blood passes from one twin to the other through a shared placenta.

Ultrasound: A test that uses sound waves to examine the fetus.

This Patient Education Pamphlet was developed under the direction of the Committee on Patient Education of the American College of Obstetricians and Gynecologists. Designed as an aid to patients, it sets forth current information and opinions on subjects related to women's health. The average readability level of the series, based on the Fry formula, is grade 6–8. The Suitability Assessment of Materials (SAM) instrument rates the pamphlets as "superior." To ensure the information is current and accurate, the pamphlets are reviewed every 18 months. The information in this pamphlet does not dictate an exclusive course of treatment or procedure to be followed and should not be construed as excluding other acceptable methods of practice. Variations taking into account the needs of the individual patient, resources, and limitations unique to the institution or type of practice may be appropriate.

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