How does multiple pregnancy occur?

A twin, triplet, or higher-order pregnancy (four or more babies) is called multiple pregnancy. If more than one egg is released during the menstrual cycle and each is fertilized by a sperm, more than one embryo may implant and grow in your uterus. This type of pregnancy results in fraternal twins (or more). When a single fertilized egg splits, it results in multiple identical embryos. This type of pregnancy results in identical twins (or more). Identical twins are less common than fraternal twins.

What are some causes of multiple pregnancy?

The use of fertility drugs to induce ovulation often causes more than one egg to be released from the ovaries and can result in twins, triplets, or more. In vitro fertilization can lead to a multiple pregnancy if more than one embryo is transferred to the uterus. Identical multiples also may result if the fertilized egg splits after transfer. Women older than 35 years are more likely to release two or more eggs during a single menstrual cycle than younger women. Therefore, they are more likely than younger women to become pregnant with multiples.

What are some symptoms of multiple pregnancy?

Women who are pregnant with multiples may have more severe morning sickness or breast tenderness than women who are pregnant with a single baby. They also may gain weight more quickly. Most multiple pregnancies are discovered during an ultrasound exam.

Do I need to gain extra weight if I am pregnant with multiples?

It generally is recommended that women who are pregnant with multiples gain more weight than women who are pregnant with one baby. An extra 300 calories a day is needed for each fetus. For instance, if you are pregnant with twins, you need an extra 600 calories a day. For triplets and higher-order pregnancies, weight gain should be individualized.

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Should I exercise if I am pregnant with multiples?

Staying active during multiple pregnancy is important for your health, but you may need to avoid strenuous exercise. Try low-impact exercise, such as swimming, prenatal yoga, and walking. You should aim for 30 minutes of exercise a day. If problems arise during your pregnancy, it may be recommended that you avoid exercise.

Is the risk of complications higher if I am pregnant with multiples?

The risk of certain complications is higher if you are pregnant with multiples. You most likely will have more frequent prenatal care visits with your obstetrician. Starting in your second trimester, you may have ultrasound exams every 4–6 weeks. If a problem is suspected, you may have special tests, such as a nonstress test or biophysical profile, and more frequent ultrasound exams.

What is the most common complication of multiple pregnancy?

The most common complication of multiple pregnancy is preterm birth. More than one half of all twins are born preterm. Higher-order multiples are almost always born preterm. Babies born before 37 weeks of pregnancy may have an increased risk of short-term and long-term health problems, including problems with breathing, eating, and staying warm. Other problems, such as learning and behavioral disabilities, may appear later in childhood or even in adulthood. Very preterm babies (those who are born before 32 weeks of pregnancy) can die or have severe health problems, even with the best of care.

Preterm multiples also have a greater risk than single preterm babies of the same gestational age for serious complica-tions that can lead to cerebral palsy. Children born with problems related to being preterm may need lifelong medical care.

What are chorionicity and amnionicity?

Early in a multiple pregnancy, an ultrasound exam is done to find out whether each baby has its own chorion (chorionicity) and amniotic sac (amnionicity). There are three types of twins:

  1. Dichorionic–diamniotic—Twins who have their own chorions and amniotic sacs. They typically do not share a placenta and can be fraternal or identical.
  2. Monochorionic–diamniotic—Twins who share a chorion but have separate amniotic sacs. They share a placenta and are identical.
  3. Monochorionic–monoamniotic—Twins who share one chorion and one amniotic sac. They share a placenta and are identical.

What are the risks associated with monochorionic babies?

Monochorionic babies have a higher risk of complications than those with separate placentas. One problem that can occur in monochorionic–diamniotic babies is twin–twin transfusion syndrome (TTTS). In TTTS, the blood flow between the twins becomes unbalanced. One twin donates blood to the other twin. The donor twin has too little blood, and the recipient twin has too much blood. The earlier TTTS occurs in the pregnancy, the more serious the outcomes for one or both babies.

Although monochorionic–monoamniotic babies are rare, this type of pregnancy is very risky. The most common problem is an umbilical cord complication. Women with this type of pregnancy are monitored more frequently and are likely to have their babies by cesarean delivery.

How can multiple pregnancy affect my risk of preeclampsia?

Preeclampsia is a blood pressure disorder that usually starts after 20 weeks of pregnancy or after childbirth. It occurs more often in multiple pregnancies than in singleton pregnancies. It also tends to occur earlier and is more severe in multiple pregnancies. Preeclampsia can damage many organs in your body, most commonly your kidneys, liver, brain, and eyes. Preeclampsia that worsens and causes seizures is called eclampsia. When preeclampsia occurs during pregnancy, the babies may need to be delivered right away, even if they are not fully grown.

How can multiple pregnancy affect my risk of gestational diabetes?

Women carrying multiples have a high risk of gestational diabetes. This condition can increase the risk of preeclampsia and of developing diabetes later in life. The newborns may have breathing problems or low blood sugar levels. Diet, exercise, and sometimes medication can reduce the risk of these complications.

How can multiple pregnancy affect fetal growth?

Multiples are more likely to have growth problems than single babies. Multiples are called discordant if one fetus is much smaller than the others. Discordant growth is common with multiples. It does not always signal a problem. Sometimes, though, a fetus’s restricted growth may be caused by an infection, TTTS, or a problem with the placenta or umbilical cord. If growth restriction is suspected in one or both babies, frequent ultrasound exams may be done to track how the babies are growing.

Are tests for genetic disorders as accurate in multiple pregnancies?

Screening tests for genetic disorders that use a sample of the mother’s blood (serum screening tests) are not as sensitive in multiple pregnancy. It is possible to have a positive screening test result when no problem is present in either baby.Diagnostic tests for birth defects include chorionic villus sampling and amniocentesis. These tests are harder to perform in multiples because each fetus must be tested. There also is a small risk of loss of one or all of the fetuses. Results of these tests may show that one fetus has a disorder, while the others do not.

How can multiple pregnancy affect delivery?

The chance of needing a cesarean delivery is higher with multiples. In some cases, twins can be delivered by vaginal birth. How your babies are born depends on the following:

  • Number of babies and the position, weight, and health of each baby
  • Your health and how your labor is going
  • Your obstetrician’s experience

Can multiple pregnancy affect my risk of postpartum depression?

Having multiples might increase your risk of postpartum depression. If you have intense feelings of sadness, anxiety, or despair that prevent you from being able to do your daily tasks, let your obstetrician or other member of your health care team know.

Can I breastfeed if I have multiples?

Yes, but it may take some practice. Your milk supply will increase to the right amount. You will need to eat healthy foods and drink plenty of liquids. Lactation specialists are available at many hospitals and in your community to help you work out any problems you may have.


Amniocentesis: A procedure in which a needle is used to withdraw and test a small amount of amniotic fluid and cells from the sac surrounding the fetus.

Amnionicity: The number of amniotic (inner) membranes that surround babies in a multiple pregnancy. When multiples have only one amnion, they share an amniotic sac.

Amniotic Sac: Fluid-filled sac in the mother’s uterus in which the fetus develops.

Biophysical Profile: An assessment of fetal heart rate by electronic fetal monitoring and assessment of fetal breathing, body movement, muscle tone, and the amount of amniotic fluid by ultrasound. The biophysical profile can be modified to include only some of these tests.

Cerebral Palsy: A long-term disability of the nervous system that affects young children in which control of movement or posture is abnormal and is not the result of a recognized disease.

Cesarean Delivery: Delivery of a baby through surgical incisions made in the mother’s abdomen and uterus.

Chorion: The outer membrane that surrounds the fetus.

Chorionic Villus Sampling: A procedure in which a small sample of cells is taken from the placenta and tested.

Chorionicity: The number of chorionic (outer) membranes that surround the fetuses in a multiple pregnancy.

Diagnostic Tests: Tests that look for a disease or cause of a disease.

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

Eclampsia: Seizures occurring in pregnancy or after pregnancy and linked to high blood pressure.

Embryo: The developing organism from the time it implants in the uterus up to 8 completed weeks of pregnancy.

Fetus: The developing organism in the uterus from the ninth week of pregnancy until the end of pregnancy.

Fraternal Twins: Twins that have developed from two fertilized eggs that are not genetically identical.

Genetic Disorders: A term for disorders caused by a change in genes or chromosomes.

Gestational Age: The age of a pregnancy calculated from the number of weeks that have elapsed from the first day of the last normal menstrual period.

Gestational Diabetes: Diabetes that arises during pregnancy.

Identical Twins: Twins that have developed from a single fertilized egg that are usually genetically identical.

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 transferred to the woman’s uterus to achieve a pregnancy.

Multiple Pregnancy: A pregnancy in which there are two or more fetuses.

Nonstress Test: A test in which changes in the fetal heart rate are recorded using an electronic fetal monitor.

Ovaries: The paired organs in the female reproductive system that contain the eggs released at ovulation and produce hormones.

Ovulation: The release of an egg from one of the ovaries.

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

Postpartum Depression: Intense feelings of sadness, anxiety, or despair after childbirth that interfere with a new mother’s ability to function and that do not go away after 2 weeks.

Preeclampsia: A disorder that can occur during pregnancy or after childbirth in which there is high blood pressure and other signs of organ injury, such as an abnormal amount of protein in the urine, a low number of platelets, abnormal kidney or liver function, pain over the upper abdomen, fluid in the lungs, or a severe headache or changes in vision.

Preterm: Born before 37 weeks of pregnancy.

Screening Tests: Tests that look for possible signs of disease in people who do not have symptoms.

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

Ultrasound Exam: A test in which sound waves are used to examine internal structures. During pregnancy, it can be used to examine the fetus.

Umbilical Cord: A cord-like structure containing blood vessels that connects the fetus to the placenta.

Uterus: A muscular organ located in the female pelvis that contains and nourishes the developing fetus during pregnancy.

If you have further questions, contact your obstetrician–gynecologist.

Designed as an aid to patients, this document sets forth current information and opinions related to women’s health. The information 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.