Breastfeeding is widely known to suppress fertility, serving as a natural form of family spacing. However, when the reproductive system begins to restart while lactation is ongoing, a unique hormonal environment is created. This transitional phase of partial fertility restoration is hypothesized to trigger a mechanism that momentarily increases the likelihood of releasing two eggs instead of one, leading to an elevated rate of twin conceptions.
Breastfeeding and Reproductive Suppression
The reproductive cycle is suppressed during intense breastfeeding, a phenomenon known as Lactational Amenorrhea. Frequent stimulation of the nipple by the infant’s suckling causes the pituitary gland to release high levels of Prolactin, the primary hormone responsible for stimulating milk production.
These elevated Prolactin levels actively interfere with the body’s regular ovulatory cycle. High Prolactin suppresses the release of Gonadotropin-releasing Hormone (GnRH) from the hypothalamus, which is the master signal for reproduction. This suppression prevents the pituitary from releasing the two hormones required for ovulation, Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH). As a result, the ovaries remain inactive, ovulation is prevented, and the return of the menstrual period is delayed.
The Biological Basis of Increased Twinning
The increased chance of conceiving twins occurs during the delicate period when fertility is beginning to return, not during full reproductive suppression. This transition happens as the frequency or intensity of suckling decreases, perhaps due to the baby starting solid foods or sleeping for longer stretches at night. When suckling becomes less constant, Prolactin levels drop but often do so inconsistently, creating a hormonal environment that is no longer fully suppressive.
This fluctuating hormonal state can cause a dysregulated signal from the brain to the ovaries. The ovaries, which have been largely dormant, may receive a surge of FSH and LH that is strong enough to stimulate follicular development but disorganized enough to overstimulate them. Instead of the usual single dominant follicle maturing and releasing one egg, the ovaries may prematurely mature and release two separate eggs in a process called hyperovulation.
The release of two eggs simultaneously is the direct biological cause of conceiving fraternal twins. This effect is theorized to be a temporary hormonal ‘rebound’ as the reproductive system attempts to fully restore its normal cyclic function. One study reported that the rate of twins among women who conceived while breastfeeding was significantly higher, approximately 11.4%, compared to the general population rate, which is about 1.1%.
Twinning Types and Other Influencing Factors
This proposed mechanism of hyperovulation only influences the conception of Dizygotic, or fraternal, twins, which develop when two separate eggs are fertilized by two separate sperm. These twins are genetically distinct, sharing only about 50% of their genes, similar to non-twin siblings. The hormonal fluctuations associated with the end of lactational amenorrhea directly create the potential for this double-ovulation event.
In contrast, Monozygotic, or identical, twins result from a single fertilized egg that spontaneously splits into two embryos very early in development. The cause of monozygotic twinning is considered a random event unrelated to maternal hormones or external factors. Therefore, breastfeeding’s effect on twinning is specific to the increased rate of fraternal twins.
Other factors also influence the likelihood of conceiving fraternal twins. Maternal age is a significant factor, as women over 35 naturally have higher levels of FSH, increasing the chance of hyperovulation. A family history of fraternal twins on the mother’s side suggests a genetic predisposition to releasing multiple eggs. Additionally, higher parity (having had multiple previous births) and certain nutritional factors may also play a role in the overall twinning rate.