Most twin pregnancies are not planned to be twin pregnancies. They happen to people who were hoping for one baby and found out, usually at an ultrasound, that they were getting two. But for some people, twins are the goal. Maybe you have twins in your family, or you are pursuing fertility treatment and want to understand your odds, or you are just curious what actually shifts the likelihood.
Here is what the research says about the factors that genuinely influence your chances of conceiving twins.
One important distinction up front: everything below applies to fraternal twins, not identical twins. Fraternal twins come from two separate eggs fertilized by two separate sperm, and a range of factors can affect how likely that is to happen. Identical twins form when a single fertilized egg splits, and that process is not thought to be influenced by any of the factors below. If you want identical twins specifically, there is no known way to increase those odds.
Genetics
The most reliable predictor of spontaneous twin conception is whether twins run in your family on your mother’s side.
The reason is hyperovulation, which is the tendency to release more than one egg in a cycle. That trait is heritable, passed mainly through the maternal line, and it is the primary driver of fraternal twin conception. If your mother, maternal grandmother, or another close female relative on your mother’s side had fraternal twins, your odds of hyperovulating are higher than average.
In 2016, a large genomic study published in the American Journal of Human Genetics analyzed data from approximately 90,000 Icelandic women and identified two specific gene variants linked to spontaneous fraternal twinning. One variant in the FSHB gene, which governs follicle-stimulating hormone production, was associated with an 18% higher relative probability of having fraternal twins. A second variant in the SMAD3 gene, involved in ovarian response to hormonal signals, was associated with a 9% higher relative probability. Women who carried both variants showed a combined effect of about 29% higher relative odds compared to women with neither.
These numbers are relative, not absolute. The baseline rate of spontaneous fraternal twins is roughly 1 in 60 pregnancies, so a 29% relative increase brings that to something like 1 in 47. It matters, but it does not make twins a likely outcome by any stretch.
Genetics on your father’s side, for the record, does not influence your odds of having fraternal twins, though he can pass the relevant genes to daughters who might then carry them.
Age
This one surprises people: your odds of conceiving fraternal twins naturally go up as you get older, at least through your mid-to-late thirties.
The mechanism is FSH, or follicle-stimulating hormone. FSH is what prompts the ovaries to develop and release eggs each cycle. As women age and ovarian reserve naturally declines, the body responds by producing more FSH in an attempt to stimulate follicle development. That elevated FSH can sometimes push the ovaries to release more than one egg, which increases the chance of fraternal twins.
Research published in the American Journal of Obstetrics and Gynecology found that the rate of spontaneous twinning peaks for women in their late thirties, with women around 35 roughly four times more likely to conceive fraternal twins than women in their early twenties. The rate climbs through the mid-thirties, peaks around age 37, and then begins to decline again as overall fertility drops.
The catch is that increased FSH also signals declining ovarian reserve, and overall fertility does decline with age. You are more likely to conceive twins spontaneously at 37 than at 27, but you are also less likely to conceive at all. Higher twin odds do not translate to better odds of a successful pregnancy.
BMI
Body mass index has a documented, if modest, association with fraternal twin conception. A higher BMI appears to correlate with higher circulating levels of IGF-1, a hormone that may increase ovarian sensitivity to FSH and promote the development of multiple follicles.
Research published in the American Journal of Obstetrics and Gynecology found that women with a BMI above 30 were approximately 1.5 times more likely to conceive fraternal twins spontaneously than women with a lower BMI, in pregnancies that did not involve fertility treatment.
This is worth knowing, but it is not a reason to try to gain weight in hopes of twins. A higher BMI also comes with increased risks during pregnancy, including gestational diabetes, hypertension, and preeclampsia, all of which are already more common in twin pregnancies. The association is informational, not a recommendation.
Race and Ethnicity
Twin rates differ across racial and ethnic groups, for reasons that are not fully understood but likely reflect a combination of genetics and access to fertility treatment.
According to data from the March of Dimes and the CDC, Black women in the United States have the highest spontaneous twin rate among major demographic groups, at approximately 42.7 twin births per 1,000 live births. White women have a rate of about 32.6 per 1,000. Hispanic women have the lowest measured rate among these groups, at approximately 24.6 per 1,000.
These differences reflect population-level patterns in hyperovulation rates, not individual outcomes. Race and ethnicity are not something you can change or act on, and this data is more useful for understanding why twin rates vary than for predicting what your own odds are.
Parity
Parity refers to the number of pregnancies you have carried. Research consistently shows that women who have had previous pregnancies are more likely to conceive fraternal twins than women who have never been pregnant. The reasons are not entirely clear, though one theory is that repeated pregnancies may be associated with slightly elevated FSH levels over time.
This is a modest effect, and not a reason to have more children than you want. But if you have had previous pregnancies and are wondering about your twin odds, parity is a real factor working in the background.
Breastfeeding
This one is genuinely surprising. A study published in the Journal of Reproductive Medicine found that women who conceived while breastfeeding had a twin rate of 11.4%, compared to 1.1% in women who were not breastfeeding. The proposed mechanism is that breastfeeding elevates prolactin and suppresses normal ovulation, which may cause hormonal fluctuations that prompt the release of more than one egg when ovulation does resume.
The sample size in that study was relatively small, and this finding has not been widely replicated. It is interesting and worth knowing, but not something to treat as a reliable strategy. Conceiving while breastfeeding is also more complicated than it sounds, since breastfeeding is itself a form of fertility suppression for many women.
Fertility Treatment
If you are looking for the factor with the largest actual effect on twin odds, this is it.
Ovulation-stimulating medications, particularly clomiphene citrate (Clomid) and injectable gonadotropins, work by stimulating the ovaries to develop multiple follicles. That substantially increases the chance of releasing more than one egg and conceiving fraternal twins or higher-order multiples. The twin rate among women using gonadotropins without IVF can be as high as 20 to 25%.
IVF increases twin odds primarily through the transfer of more than one embryo. Historically, transferring two embryos was common practice, and twin rates among IVF patients were high as a result. In recent years, many clinics have shifted toward single embryo transfer whenever possible, especially for younger patients with good embryo quality, specifically because of the higher risks associated with twin pregnancies. IVF still results in more twins than natural conception, but the rates vary significantly based on how many embryos are transferred and what your clinic’s protocol is.
If you are undergoing fertility treatment and asking about twin odds, your reproductive endocrinologist is the right person to talk to. The answer depends on your specific protocol, your age, your embryo quality, and your clinic’s approach to embryo transfer.
What This Adds Up To
The factors that meaningfully increase spontaneous twin odds are: having hyperovulation genes in your family (particularly your mother’s side), being in your mid-to-late thirties, having a higher BMI, having had previous pregnancies, and being a Black woman in the United States. None of these are things most people can or should try to engineer. They are background conditions that shift the odds in ways you may or may not have control over.
Fertility treatment is the one lever with a large and predictable effect, and decisions about that belong in a conversation with your doctor.
For most people asking this question, twins are not something you can reliably plan for. What happens will mostly happen on its own terms. But understanding the biology of why some people are more likely to have twins is genuinely interesting, and if any of these factors apply to you, it is worth knowing what they mean.
Sources
- Identification of Common Genetic Variants Influencing Spontaneous Dizygotic Twinning and Female Fertility. American Journal of Human Genetics, 2016. Cell AJHG S0002-9297(16)30147-X
- High rate of dizygotic twinning with older maternal age. American Journal of Obstetrics and Gynecology, 2004. PubMed 15531775
- Body mass index and the risk of dizygotic twinning. American Journal of Obstetrics and Gynecology, 2004. PubMed 15531775
- Spontaneous twinning rates around the world. Human Reproduction, 2006. PubMed 16006453
- Twins, Triplets, and Other Multiples. March of Dimes / CDC. marchofdimes.org
- Breast-feeding and spontaneous twinning. Journal of Reproductive Medicine, 2006. PubMed 16548063
- Ovarian stimulation and multiple pregnancies. New England Journal of Medicine, 2004. NEJM 10.1056/NEJMra035092
