Twins and Multiples

Twins are the result of either the splitting of a single fertilized egg or the parallel development of two fertilized eggs. Two out of three sets of twins are fraternal (not identical), and one third of twins conceived naturally are identical. In 1980, one out of 50 newborns was a twin, but that number is now one out of 30. Forty percent of all twins in the US are conceived either through IVF or some other fertility treatment. Older women are more likely to conceive multiples, regardless of whether infertility treatments are used to accomplish the goal of conception. All women, however, are capable of conceiving multiples.

Most multiple gestation pregnancies involve risks and complications that can affect babies and mother alike, and should therefore be closely monitored by a physician. Women who have previously been pregnant, have recently discontinued birth control, or have a higher body mass index (BMI) are more likely to have multiple pregnancies. A family history of twins also increases the chances of having multiples.

If you are expecting twins, prepare to receive more extensive care. Your doctor will want to see you more often and may run more tests to ensure that you and your fetuses are doing well. Ask your doctor or healthcare provider about classes for parents who are expecting multiples. Your doctor will arrange additional prenatal visits and advise you to get plenty of rest and take extra vitamins and supplements during your pregnancy.

Identical vs. Fraternal

Monozygotic (identical) and Dizygotic (fraternal) pregnancies occur most commonly as a result of infertility treatments. Identical fetuses come from a single egg and are genetically identical, with similar features, the same gender, and identical DNA. They often share one placenta (but not always) and are the result of one fertilized egg splitting into two embryos. Only about one in 300 spontaneously conceived pregnancies are identical twins, and things like race, heredity, and age do not influence this random event.

Fraternal twins occur when multiple eggs are released and fertilized, and each twin gets its nutrition from his or her own placenta. Such twins are not necessarily the same gender, and they usually don’t look alike at birth. Even though they were carried during the same pregnancy, they are no more genetically similar than normal siblings.

Identical Twins: Timing of the Splitting of a Fertilized Egg

The type of identical twin pregnancy depends upon how soon after fertilization the embryo splits. A very early split (before day 4) can lead to two separate embryos, two placentas, and basically no way to determine that the twins are identical unless DNA testing is done. Some identical twins grow up being told that they were fraternal because in the past even medical professionals did not always appreciate this aspect of identical twinning.

Usually the fertilized egg splits later (four to eight days after fertilization), producing one placenta that both babies share; fortunately, however, they each get their own amniotic sac. This is called monochorionic (one placenta), diamniotic (two sacs) twinning. If the embryo splits later (days 8–12), an even rarer condition occurs, which is when both babies share the same amniotic sac. This is referred to as monoamniotic twins. This is a very dangerous situation for the babies because during pregnancy, both fetuses move so much that invariably their umbilical cords get tangled up, which in many cases causes them to pass away while still inside the womb.

There is one more extremely rare situation: when the fertilized egg splits even later (day 13 or after). This is how conjoined twins (Siamese twins) develop. The fertilized egg (now a tiny embryo) splits so late that the two portions do not completely separate, leading to twins that share part of their body with their other twin.

Higher Order Multiples

Higher order multiples include sets of triplets, quadruplets, quintuplets, etc. Variations of identical and fraternal multiples can and will occur. It is not uncommon for a set of triplets to include one fraternal triplet while the other two are identical. In the United States, triplets account for one in every 8,000 live births.

[Editor’s note: Dr. Jick knows someone who gave birth to identical triplets, considered to be about a one in one million occurrence]

Signs That Indicate You May Be Having Twins

Given that most pregnant women have an ultrasound early in pregnancy, the days of the surprise discovery of twins late in pregnancy (or rarely, at delivery) are long past. However, in case there is no access to early OB ultrasound, this information might be useful.

A woman who is pregnant with more than one fetus may experience rapid weight increases during the first trimester. During routine examinations the doctor may notice the uterus is much larger than expected, which is a good sign that multiple fetuses may be present. An ultrasound can confirm this if one has not yet been done. Higher hCG levels and more intense or severe pregnancy symptoms — nausea, vomiting, and breast tenderness — are also indicators there may be more than one fetus.

Staying Healthy While Pregnant with Multiples

When you are pregnant with multiples you are typically not any hungrier than you would be if you were pregnant with one fetus. However, you will need to eat more than if you were eating for one baby. Eating healthy and gaining a healthy amount of weight while pregnant with multiples is critical to your health and that of your babies. For women who are average weight, eating 2,700 calories each day — or an additional 300 calories per fetus — is recommended.

When it comes to gaining weight, plan on gaining 35 to 45 pounds for the entire twin pregnancy. During the first half of your pregnancy, you should try to gain an average of one pound per week when carrying twins. During the second half, you should try to gain more, especially if you were underweight before conception. If you were overweight before conception, you may want to gain less. Ask your healthcare provider how much weight you should gain each week.

Anemia (a lack of iron in your blood) is commonly seen in women who are pregnant with multiples. Typically, your intake of iron, folic acid, and other vitamins and minerals should be increased throughout your pregnancy. Iron is better absorbed when you eat it with foods that are high in acid, like yogurt, or foods that are high in vitamin C, like oranges. Besides iron and folic acid, you should also increase your intake of copper, zinc, Vitamins D and C and Calcium.

TIP: Do not increase the number of prenatal vitamins you take simply because you are carrying more than one baby. One prenatal vitamin a day is sufficient; more than that can be harmful. It is better to take one prenatal vitamin daily plus additional supplements as needed.

Complications of Twins

A multiple pregnancy is generally safe for both Mom and babies as long as the mother receives adequate prenatal care, rest, and good nutrition. Pregnancies with more than one fetus tend to last 35 to 37 weeks, which is a bit less than the normal 40-week gestation period. Your doctor may want to see you more often during your pregnancy than if you were only carrying one baby, in order to ensure that everything is okay.

Incompetent cervix, premature labor, premature birth, premature rupture of the membranes, anemia, high blood pressure, pre-eclampsia, gestational diabetes, placenta previa, postpartum bleeding, and low birth weight are all possible complications mothers and babies can experience before and after birth. Twins are a high-risk pregnancy, and people expecting twins should consider looking for a care provider with special expertise in the care of twin pregnancies.

Preparing for Delivering Multiples

Preparing for the birth of your children can be an overwhelming task. If you feel overwhelmed, talk with your partner, family, and friends about it. You can also talk to your doctor or healthcare provider, who may be able to refer you to a support group or class designed to prepare parents who are expecting multiples.

You will also want to talk with your doctor about the benefits and risks of vaginal versus C-section methods of delivery. Depending on your health and the health of your babies, you may not have a choice in how your babies are delivered. If your doctor feels a vaginal birth would be too risky, he or she may recommend a C-section procedure. In fact, twins or multiples are delivered via C-section more often than vaginally, especially if one or more of the babies are in breech position (feet or buttocks first). If the first baby is in breech position, a cesarean should be performed. In some cases, one baby may be delivered vaginally and the other by (unplanned) cesarean. If both babies are head down and close to term, safe vaginal birth may be possible. For triplets or more, a C-section is usually the method of delivery for all. Your safety and that of your babies take priority over delivery preferences.


In most cases, you will be united with your babies shortly after giving birth. Multiples born prematurely are typically taken to the NICU (Neonatal Intensive Care Unit) immediately for special care.

In the days, weeks, and months after delivery, most new parents of multiples find themselves incredibly overwhelmed by the lack of sleep, frequent feedings, and decreased personal time. It is best to have help whenever possible as you make the transition into parenthood. Family and friends can help you through the transition.

Advice: For the first six weeks, it takes 1.5 people to care for one newborn

Dr. Jick discusses with his patients his principle of 1.5 people to take care of one baby. For a normal couple (Mom and Dad for purposes of discussion) bringing home one baby, the mom gets a score of 0.5 for the first two weeks, since she is recovering from her childbirth and the dad (who is hopefully planning to stay home full-time for the first two weeks) gets a score of 1. This means that a mom and dad can normally take care of a single newborn 24/7 for the first six weeks. This does not mean it is easy, but usually it is doable.

After two weeks, the mom has recovered somewhat and is home full-time, so she gets a score of 1, and the dad returns to work, so he gets the 0.5 score. This is obviously not one-size-fits-all description, but it can be useful for making plans.

If Mom and Dad are bringing home twin newborns, then 1.5 people per baby means that three able-bodied people are needed around the clock 24/7 for the first six weeks! Mom gets a score 0.5 and Dad gets a score of 1, so my advice to parents is to arrange 1.5 more people to help take care of those twins for the first six weeks. This usually means one live-in family member (such as a mom, aunt, sister, or cousin) if at all possible, plus some part-time help as well, such as a paid housekeeper (or awesome friends) who can help with chores, laundry, grocery shopping, cooking, cleaning, and other non-baby-related activities.

The above is not meant to imply that two healthy people with newborn twins are incapable of taking care of them all by themselves 24/7—sometimes there is no choice. But if you have a choice, and if you have friends or relatives available to help, this can make a huge difference in your physical recovery, your milk production, and your mental and emotional health. Try to bring your caregiver score close to that 3.0 number for the first six weeks with those beautiful, hungry, and messy twin newborns.

This page was last updated on 06/2017

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