How to have twins: Factors, odds, and improving your chances

Twins can occur either when two separate eggs become fertilized in the womb or when a single fertilized egg splits into two embryos.

Having twins is more common now than it was in the past. According to the Centers for Disease Control and Prevention (CDC), twin births have nearly doubled over the last 40 years.

A woman has a higher chance of having twins if she conceives with the assistance of fertility treatment or is aged 35 years or over. In the United States, fertility treatments account for more than one-third of twin pregnancies.

In this article, we explore why twin pregnancies occur, how common they are, and factors that can make them more likely. We also explain whether a person can increase their chance of having twins.

Why do twin pregnancies happen?

Doctors do not fully understand the reasons why twin pregnancies sometimes occur. However, some factors can increase the likelihood of giving birth to twins, including:

  • the woman’s age
  • having a family history of twins
  • having fertility treatments

Conception happens when a sperm fertilizes an egg to form an embryo. However, if there are two eggs present in the womb at the time of fertilization or the fertilized egg splits into two separate embryos, a woman can become pregnant with twins.

There are two types of twin:

  • Identical twins: This type of pregnancy happens when a fertilized egg splits into two separate embryos. These embryos are monozygotic, which means that they have identical genes. Identical twins are the same sex as each other and look very alike.
  • Nonidentical, or fraternal, twins: This type of pregnancy occurs when there are two eggs present in the womb at the time of fertilization, and sperm fertilize both of them. These embryos are dizygotic, which means that they do not have identical genes and may not be the same sex.

Fraternal twins are common after fertility treatment because healthcare professionals often place two fertilized embryos into a woman’s womb to increase the chances of a successful pregnancy.

There are many unproven claims about how to increase the likelihood of conceiving twins. Some people advise following specific diets or using certain alternative therapies, but there is no scientific evidence to support these methods.

Fertility treatments, particularly IVF and ovary stimulants, do increase the chances of having twins. However, a twin pregnancy is riskier for both the woman and the developing fetuses. For this reason, some fertility clinics advise against implanting multiple embryos during IVF treatment.

The CDC recommend that younger women who are having their first fertility treatment select just one embryo for transfer to their womb.

As the success of fertility treatment is improving, there is often less need to transfer more than one embryo. In the U.S., the number of IVF treatments involving the transfer of three or four embryos decreased significantly between 2007 and 2016. Transferring just one or two embryos reduces the likelihood of a multiple pregnancy.

A twin pregnancy increases the risk of:

  • premature birth
  • low birth weight
  • stillbirth
  • disabilities at birth and congenital health conditions, such as cerebral palsy and autism
  • preeclampsia
  • diabetes during pregnancy
  • cesarean delivery
  • needing bed rest while pregnant


Although certain factors increase the chance of having twins, there is no way to improve the odds of having twins naturally. A woman will usually find out that she is pregnant with twins from an ultrasound early on in the pregnancy.

Some symptoms may indicate a twin pregnancy, including more intense morning sickness and very rapid weight gain. Regular medical appointments will improve the chances of a healthy pregnancy.

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