Surprise! Twins on the Way!
Planning a pregnancy? If you’re over 35, especially if you’re considering fertility treatments, there’s a good chance you could be expecting twins…or even triplets!
While global birth rates are generally declining, the number of multiple births is rising. According to the American Society for Reproductive Medicine (ASRM), approximately 33 out of every 1,000 births in the U.S. result in twins. (1)
Why Are Multiple Births More Common Today?
What’s behind this trend? As reported in Population and Development Review, a major factor is the increased use of fertility treatments, such as IVF. These procedures often involve transferring more than one embryo to improve success rates, which in turn, raises the likelihood of having multiples. In addition, more women are choosing to have children later in life, and age itself is a contributing factor to the chances of conceiving twins or more. (2)
But that’s not the only reason. The ASRM also highlights other influences—such as genetics, ethnicity, geographic region, and previous pregnancies—that can affect the chances of having fraternal twins, which occur when two separate eggs are fertilized. (1)
Unlike identical twins, which happen randomly, fraternal twins are influenced by hereditary and biological factors. For example, studies show that black and non-Hispanic white women have similar rates of twinning, whereas the rate tends to be lower among Hispanic women. (1)
Family history is another key element: A woman who’s a fraternal twin has about a 1 in 60 chance of having twins herself, while a man who’s a fraternal twin has about a 1 in 125 chance of fathering twins. (1)
Infographic design by Boris Benko
Taking a Closer Look at Historical Twin Data and Probabilities
Based on ASRM’s publication about multiple pregnancies, age and previous childbirths also play a role. Women aged 35 to 40 who have had four or more children are nearly three times as likely to conceive twins compared to women under 20 with no prior births. (1)
Fraternal twins also appear more frequently in taller or larger women, possibly due to better nutrition rather than body size alone. Historical trends support this: During food shortages in World War II, the rate of fraternal twin births noticeably declined in parts of Europe. (1)
Certain fertility medications that stimulate ovulation can also increase the likelihood of multiple births. For instance, 5% to 12% of women who take clomiphene citrate may conceive twins, while fewer than 1% may have triplets or more. Gonadotropins, another type of fertility treatment, lead to multiples in about 30% of cases. (Mostly twins, though higher-order multiples are possible if multiple eggs are released during ovulation.) (1)
These treatments can influence the occurrence of both fraternal and identical twins.
Be Aware of the Risks
While advances in maternal care have greatly improved outcomes, carrying multiples still comes with added risks. Pregnancies involving twins or more are more likely to lead to complications, such as preeclampsia, gestational diabetes, and postpartum hemorrhaging. (1)
Babies born as multiples, especially if they’re premature, may face challenges, like low birth weight, breathing difficulties, or developmental delays. (1)
But don’t let these risks scare you too much. If you’re graced with twins (or more), then embrace the twinning lifestyle, and count yourself lucky. Not everyone gets to have the joy of raising multiple babies!
FAQ
The increase over recent decades has been dramatic. Since the 1980s, the rate of twin births worldwide increased by more than 30%, rising from 9 twin births per 1,000 deliveries in 1980–1985 to 12 per 1,000 in 2010–2015. The absolute number of twin deliveries rose 42% during this period, from 1.1 million in the early 1980s to 1.6 million in the early 2010s, while the total number of births worldwide increased by only 8% in the same period. In the U.S., the twin birth rate rose by 76% between 1980 and 2009, meaning that in 1980 one in every 53 babies born was a twin, compared with one in every 30 births in 2009. However, the trend may be leveling off: researchers suggest the world may have reached “peak twin,” as the most recent data indicate the rate is no longer continuing to climb.
The distinction comes down to how they form in the womb. Identical twins, known as monozygotic, form from a single fertilized egg that splits into two separate embryos, so they have nearly the same DNA and are always the same sex. Fraternal twins, known as dizygotic, result from two separate eggs fertilized by two separate sperm, making them no more genetically alike than siblings born at different times. Fraternal twins can be the same sex or different sexes.
Crucially, only one of these types is driving the twin boom. The rate of monozygotic (identical) twinning has remained relatively constant worldwide at about three to four per 1,000 births, meaning the vast majority of the global increase is due to a surge in dizygotic (fraternal) twins.
Age changes how a woman’s body releases eggs. After age 30, a woman’s ovaries are more likely to release more than one egg during a menstrual cycle, which increases the chance of fraternal twins even without fertility treatment. Women aged 35 to 40 who have had four or more children are nearly three times as likely to conceive twins compared to women under 20 with no prior births.
IVF raises twin rates primarily through how many embryos are transferred at once. Transferring two embryos into the uterus results in a twin pregnancy rate of about 25–30%, versus less than 1% if one embryo is transferred. In the United States, approximately 30% of IVF pregnancies result in twins, and another 3–4% result in triplets or higher-order multiples. To reduce these rates, many clinics now recommend elective single-embryo transfer (eSET): twin rates after eSET fall to around 1–2%, and freezing extra embryos for later cycles can achieve comparable overall pregnancy rates without the risks associated with twinning.
Yes, and the odds vary significantly by drug. Fertility medications that stimulate ovulation can increase the likelihood of multiple births: 5–12% of women who take clomiphene citrate may conceive twins, while fewer than 1% may have triplets or more. Gonadotropins, another type of fertility treatment, lead to multiples in about 30% of cases.
It does, but only for fraternal twins, and the effect is stronger on the mother’s side. A woman who is herself a fraternal twin has about a 1 in 60 chance of having twins, while a man who is a fraternal twin has about a 1 in 125 chance of fathering twins. This is because the hereditary factor relates to the tendency to release multiple eggs during ovulation, something only the mother’s biology directly determines.
Twin pregnancies carry meaningfully higher medical risks than singleton pregnancies. Preeclampsia complicates 10–20% of multiple gestations, two to five times higher than in singleton pregnancies, and in twin pregnancies it is often more severe and presents earlier. Pregnancies involving twins are also more likely to lead to gestational diabetes and postpartum hemorrhaging, and babies born as multiples may face challenges such as low birth weight, breathing difficulties, or developmental delays, particularly if they arrive prematurely.
