Having twins is an exciting possibility for many expecting parents. While the chances of conceiving twins naturally is about 3 in 100 pregnancies, there are several factors that can increase a woman’s likelihood of carrying twins. Understanding what influences twin conception can help couples who hope to have twins or want to avoid a twin pregnancy. Here we’ll explore the key factors that raise the odds of having fraternal (dizygotic) and identical (monozygotic) twins.
Maternal Age
A mother’s age is one of the biggest influences on her chances of having twins. As women get older, they release more follicle stimulating hormone (FSH) and are more likely to release multiple eggs during ovulation. Each released egg has the potential to be fertilized and develop into a separate fetus.
Women over 35 have the highest rates of twinning. According to the Centers for Disease Control and Prevention (CDC), a woman in her 30s has a 1 in 50 chance of conceiving twins. After age 40, the odds increase to more than 1 in 10. The rates of fraternal twinning by maternal age are:
- Under age 20: 1 in 200
- Age 20-24: 1 in 125
- Age 25-29: 1 in 80
- Age 30-34: 1 in 60
- Age 35-39: 1 in 50
- Age 40-44: 1 in 35
- Age 45-49: 1 in 20
The greater frequency of twinning with advanced maternal age is believed to explain much of the recent rise in twin birth rates in developed countries where more women are having children later in life.
Family History
A family history of twins significantly increases a woman’s chances of having her own twins. Having a twin sibling or parent makes you 2-3 times more likely to conceive twins.
This is because of genetic factors that get passed down through families. Some women inherit genes from their father that make them more prone to release multiple eggs during ovulation. Fraternal twins happen when two eggs are fertilized at the same time, so these inherited genes boost the odds of twinning.
If twins run in the family, a woman has a higher probability of hyperovulation (releasing multiple eggs). Women with a twin sibling have roughly double the chance of twins compared to the average population.
Parity
The number of pregnancies a woman has carried also influences twin probabilities. Having previous pregnancies, especially multiple past pregnancies, raises the odds of conceiving twins.
Women who have been pregnant before naturally have higher levels of FSH, which as noted previously, can lead to releasing multiple eggs during ovulation. The twinning rates based on number of prior pregnancies are:
- 0 prior pregnancies: 1 in 250 chance of twins
- 1 prior pregnancy: 1 in 80 chance of twins
- 2 prior pregnancies: 1 in 65 chance of twins
- 3 prior pregnancies: 1 in 40 chance of twins
- 4+ prior pregnancies: 1 in 8 chance of twins
This parity effect interacts with maternal age as well. The highest chance of twins occurs in older women who have had multiple past pregnancies.
Ethnicity
Twinning rates also vary by ethnicity. Women of African descent are more likely to conceive twins compared to Caucasian, Asian and Hispanic women.
Here is a breakdown of twinning frequency by race in the United States according to 2009 data from the National Center for Health Statistics:
- African American: 4.1% of births were twins
- Native American: 3.8%
- Caucasian: 3.4%
- Asian: 2.6%
- Hispanic: 2.3%
The increased incidence in African Americans is believed to reflect higher rates of hereditary factors that boost hyperovulation.
Assisted Reproductive Technology (ART)
Use of infertility treatments like in vitro fertilization (IVF) dramatically raises the chances of having twins or higher-order multiples (triplets or more). On average, about 1 in 4 IVF pregnancies result in twins.
With IVF, there are two reasons twin rates are higher:
- Multiple embryos are often transferred to the uterus to improve success rates. IVF twins are fraternal twins that occur from two separate embryos.
- Fertility drugs used before IVF overstimulate the ovaries, causing them to release multiple eggs. If multiple eggs get fertilized, it results in fraternal twins.
Other assisted reproductive techniques like intrauterine insemination (IUI) with fertility medication also have significantly higher twin rates than natural conception (up to 10% for IUI).
Medications that Increase Ovulation
Certain oral medications used to induce ovulation, like Clomid and gonadotropins, can also increase the chance of conceiving twins. These fertility drugs work by stimulating the ovaries to release multiple eggs at once during ovulation.
Clomid leads to twin rates of 5-12%, while gonadotropins have even higher twinning rates of up to 30%. Gonadotropins, which are injectable hormones, tend to have stronger ovulatory effects and greater odds of hyperovulation.
Fertility Supplements
There is some research showing fertility supplements like folic acid may slightly increase the likelihood of twins. One study found women who took folic acid supplements before conception were 1.5 times more likely to have twins compared to women who didn’t take folic acid.
The mechanism behind this possible effect is unknown. However, some think folic acid may promote ovulation of multiple eggs or increase the survival of multiple embryos. More research is needed to definitively prove this relationship.
Height and Weight
Some studies have found correlations between maternal height/weight and twinning rates. Taller and heavier women tend to have slightly elevated odds for twins.
In a large study of over 2 million births, researchers found overweight women with a BMI over 30 had nearly double the chance of twins compared to women in the optimal BMI range. Obese women with a BMI above 35 tripled their odds.
It’s thought women at the extremes of height and weight may produce more GH and IGF, hormones that can increase ovarian sensitivity. However, the effects of BMI seem to be mediated primarily through ART – obese women who conceive naturally don’t appear to have increased twinning rates.
Nutrition
Eating a diet high in certain nutrients like dairy and carbohydrates is suspected to up the likelihood of conceiving twins. One study found women who ate more dairy foods were 5 times more likely to have twins compared to women who ate less dairy.
It’s hypothesized that components in dairy like growth hormones and proteins may stimulate ovarian function and increase ovulatory response. However, the link between diet and twinning is speculative and needs more research.
Breastfeeding
Some research indicates breastfeeding may slightly raise the chances of having twins in a subsequent pregnancy. It’s thought that residual hormonal effects of nursing or not resuming menstruation may increase ovarian sensitivity.
In one study, women who conceived while breastfeeding were over 5 times more likely to have twins compared to women who weaned before conceiving again. The association seems strongest with short intervals between birth and conception.
Higher Fertility
Women with better fertility in general have been shown to have somewhat elevated rates of twins. Contributing factors like optimal age and healthy lifestyles improve ovarian function and likelihood of multiple follicle development.
Being in excellent reproductive health and at peak fertility makes a woman more likely to release multiple eggs and conceive fraternal twins. Having twins is therefore viewed by some as a marker of high fertility.
Genetic Factors
Certain genetic factors passed down paternally can increase a woman’s predisposition to hyperovulation and having fraternal twins. The father’s genes contribute to ovarian sensitivity and follicular development.
In rare cases, there are specific twin gene mutations directly inherited from the father that almost guarantee twins. This includes the twinning gene on the Y chromosome, which leads to double ovulation in over 50% of cases.
Zygosity
Twinning rates are also impacted by zygosity – whether twins are fraternal (dizygotic) or identical (monozygotic). Fraternal twins occur from two separately fertilized eggs and have two separate placentas and amniotic sacs. Identical twins derive from one fertilized egg that splits into two embryos, so they share a placenta and sac.
Fraternal twins are much more common, representing about 2/3 of twins born. Having fraternal twins is predominantly influenced by the factors above like family history, maternal age and use of fertility treatments. The odds of conceiving fraternal twins ranges from 1 in 60 naturally to 1 in 4 with IVF.
On the other hand, identical twinning rates remain constant at around 3-4 per 1,000 births worldwide – or approximately 1 in 250-300 pregnancies. The rate of identical twins stays stable across age, ethnicity and other factors. It’s mainly just a random occurrence explained by early embryo division.
Uterine Factors
Some aspects of a woman’s uterus can make it more hospitable for nurturing twin pregnancies. Having a uterine anomaly like a bicornuate uterus (heart-shaped uterus) has been associated with higher twinning rates.
Women who undergo procedures like endometrial ablation that affect the uterine lining also appear to have slightly elevated twin rates after treatment. It’s possible physical or structural changes in the uterus influence implantation and embryo survival.
Summary of Main Risk Factors for Twins
Here is a quick summary of the top factors found to increase the chance of twins:
- Advanced maternal age over 35
- Family history of twins
- Being of African descent
- Having multiple previous pregnancies
- Use of fertility drugs or procedures like IVF
- Being taller or heavier than average
- Supplements like folic acid
- Breastfeeding at time of conception
- Inherited genetic predispositions
What Doesn’t Increase the Chance of Twins
While many factors influence twin rates, there are some popular theories that scientific research has disproven. Some common misconceptions include:
- Birth control pills – Stopping birth control does not increase the odds of twins in a subsequent pregnancy.
- Fertility drugs – Use of fertility medication only boosts fraternal twin chances during active treatment cycles.
- Caffeine/alcohol – No conclusive evidence these beverages impact twinning rates.
- Breast implants – Breast implants do not appear to increase hyperovulation.
- Early pregnancy ultrasound – No evidence ultrasounds split embryos and cause identical twins.
Conclusion
While most couples have no control over conceiving twins, there are a few evidenced-based steps women can take to potentially increase their chances:
- Have children earlier in reproductive years
- Gain healthy weight if under/over BMI norms
- Take folic acid supplements
- Eat more dairy and carbohydrates
- Continue breastfeeding newborns longer if hoping for twins next pregnancy
However, because these factors only slightly up the odds, there is no guaranteed way to ensure a twin pregnancy without fertility assistance. The best advice is to maintain optimal health, be aware of the main twinning factors and let nature run its course!
