If you are pregnant with more than one baby, you are far from alone. Multiple births are up in the United States. More women are having babies after age 30 and more are taking fertility drugs. Both boost the chance of carrying more than one baby. A family history of twins also makes multiples more likely.

Years ago, most twins came as a surprise. Now, most women know about a multiple pregnancy early. Women with multiple pregnancies should see their health care providers more often than women who are expecting one baby. Multiple pregnancy babies have a much higher risk of being born prematurely and having a low birth weight. There is also more of a risk of disabilities. Some women have to go on bed rest to delay labor. Finally, they may deliver by C-section, especially if there are three babies or more.

Parenting multiples can be a challenge. Volunteer help and support groups for parents of multiples can help.

If you are pregnant with more than one baby, you are far from alone. In the past two decades, the number of multiple births has climbed way up in the United States.

Information about multiples

In 2005, 133,122 twin babies and 6,208 triplet babies were born in the United States. In 1980, there were only 69,339 twin and 1,337 triplet births.

Why the increase? For one, more women are having babies after age 30. Women in their 30s are more likely than younger women to conceive more than one baby naturally. Another reason is that more women are using fertility treatments to help them conceive.

Fraternal twins are also dizygotic twins. They result from the fertilization of two separate eggs during the same pregnancy. Fraternal twins may be of the same or different sexes. They share half of their genes just like any other siblings. In contrast, twins that result from the fertilization of a single egg that then splits in two are called monozygotic, or identical, twins. Identical twins share all of their genes and are always the same sex.

Fraternal twins are also called dizygotic twins. And the difference between fraternal and identical twins is that fraternal twins derive from two different eggs. Fraternal twins may be the same gender, they may have many of the same characteristics, but also may be very different from each other and, in fact, share half of their genes just like their sisters and brothers. It's important to recognize that the difference between fraternal twins and monozygotic, or identical twins, is that monozygotic twins result from the fertilization of a single egg with a single sperm and then during the embryonic development, or during the cell splits, those massive eggs split into two individuals, which later develop into two offspring.

Identical twins are also known as monozygotic twins. They result from the fertilization of a single egg that splits in two. Identical twins share all of their genes and are always of the same sex. In contrast, fraternal, or dizygotic, twins result from the fertilization of two separate eggs during the same pregnancy. They share half of their genes, just like any other siblings. Fraternal twins can be of the same or different sexes.

Identical twins are also called monozygotic twins. They result from the fertilization of a single egg with a single sperm. And as those cells divide and multiply, at some point very early in embryonic growth they split into two individuals. So that you can imagine that the genetic material in these two identical twins is identical to each other, hence the name. The contrast to this would be fraternal twins, which are twins born at the same time. However, they are actually the result of fertilization of two different eggs, and thus can be different genders and look different, and have different characteristics. What's becoming increasingly clear is even though we call identical twins identical, and we think of their genetic material as being identical, we need to be reminded that there are environmental influences that may change, actually, the characteristics of identical twins, reminding us, as we already know, that even though identical twins come from the same genetic material, in characteristics they may actually have differences as well.

The likelihood of conceiving twins is a complex trait. It is probably affected by multiple genetic and environmental factors, depending on the type of twins. The two types of twins are classified as monozygotic and dizygotic.

Monozygotic (MZ) twins, also called identical twins, occur when a single egg cell is fertilized by a single sperm cell. The resulting zygote splits into two very early in development, leading to the formation of two separate embryos. MZ twins occur in 3 to 4 per 1,000 births worldwide. Research suggests that most cases of MZ twinning are not caused by genetic factors. However, a few families with a larger-than-expected number of MZ twins have been reported, which indicates that genetics may play a role. It is possible that genes involved in sticking cells together (cell adhesion) may contribute to MZ twinning, although this hypothesis has not been confirmed. Most of the time, the cause of MZ twinning is unknown.

Dizygotic (DZ) twins, also called fraternal twins, occur when two egg cells are each fertilized by a different sperm cell in the same menstrual cycle. DZ twins are about twice as common as MZ twins, and they are much more likely to run in families. Compared with the general population, women with a mother or sister who have had DZ twins are about twice as likely to have DZ twins themselves.

DZ twinning is thought to be a result of hyperovulation, which is the release of more than one egg in a single menstrual cycle. To explain how DZ twinning can run in families, researchers have looked for genetic factors that increase the chance of hyperovulation. However, studies examining the contributions of specific genes have had mixed and conflicting results. Few specific genes in humans have been definitively linked with hyperovulation or an increased probability of DZ twinning.

Other factors known to influence the chance of having DZ twins include the mother’s age, ethnic background, diet, body composition, and number of other children. Assisted reproductive technologies such as in vitro fertilization (IVF) are also associated with an increased frequency of DZ twins.

In another approach to gene-environment interactions, the field of epigenetics looks beyond the genotype itself and studies how the same genotype can be expressed in different ways. In other words, researchers study how the same genotype can lead to very different phenotypes. As mentioned earlier, gene expression is often influenced by environmental context in ways that are not entirely obvious. For instance, identical twins share the same genetic information (identical twins develop from a single fertilized egg that split, so the genetic material is exactly the same in each; in contrast, fraternal twins usually result from two different eggs fertilized by different sperm, so the genetic material varies as with non-twin siblings). But even with identical genes, there remains an incredible amount of variability in how gene expression can unfold over the course of each twin’s life. Sometimes, one twin will develop a disease and the other will not. In one example, Aliya, an identical twin, died from cancer at age 7, but her twin, now 19 years old, has never had cancer. Although these individuals share an identical genotype, their phenotypes differ as a result of how that genetic information is expressed over time and through their unique environmental interactions. The epigenetic perspective is very different from range of reaction, because here the genotype is not fixed and limited.

Years ago, most twins came as a surprise. Now, thanks to advances in prenatal care, most women learn about a multiple pregnancy early. You might suspect you are pregnant with multiples if you have more severe body changes, including:

• Rapid weight gain in the first trimester
• Intense nausea and vomiting
• Extreme breast tenderness

Your doctor can confirm whether you are carrying more than one baby through ultrasound. If you are pregnant with twins or other multiples, you will need to see your doctor more often than women who are carrying only one baby because your risk of complications is greater. Women carrying more than one baby are at higher risk of:

• Preterm birth
• Low birth weight
• Preeclampsia
• Gestational diabetes
• Cesarean birth

More frequent prenatal visits help your doctor to monitor your and your babies' health. Your doctor will also tell you how much weight to gain, if you need to take extra vitamins, and how much activity is safe. With close monitoring, your babies will have the best chance of being born near term and at a healthy weight.

After delivery and once your babies come home, you may feel overwhelmed and exhausted. Ask for help from your partner, family, and friends. Volunteer help and support groups for parents of multiples also can ease the transition.

If you give birth to twins or triplets, the benefits of breastfeeding to you and your children are the same as for all mothers and infants. Some women think that breastfeeding more than one infant will be overwhelming; however, it can be done with special planning and preparation.

Most mothers are able to make plenty of milk for twins. Many mothers of triplets (three infants) or quadruplets (four infants) are able to provide enough milk to breastfeed their infants completely or partially. When a mother breastfeeds partially, she supplements her breast milk with human milk from another source or with an infant formula.