The rebound effect
Hormonal contraception suppresses the hypothalamic-pituitary-ovarian axis: it keeps FSH and LH low, suppresses dominant follicle selection, and prevents ovulation. When a woman stops the pill, that axis re-engages — and it can briefly overshoot.
Several observational studies have found a small but reproducible increase in twin pregnancies in the first one to three cycles after stopping combined oral contraceptives. Murphy and colleagues, in an early Lancet paper, reported a roughly twofold increase in twin pregnancy rate among conceptions in the first cycle off the pill compared to later cycles [2]. More recent work by Yan and colleagues confirmed the pattern in a larger Chinese cohort [3].
The biology
The mechanism is short-lived FSH elevation. With prolonged ovarian suppression, FSH receptor sensitivity may be temporarily up-regulated. When endogenous FSH returns, follicular recruitment can briefly exceed the usual mono-ovulatory pattern, allowing more than one follicle to mature and ovulate. The window is narrow: by the third or fourth cycle off contraception, the rebound has dissipated and twin probability has returned to the woman’s underlying baseline.
For the calculator we apply ×1.2 if hormonal contraception was stopped within the last three months, and ×1.0 otherwise. We deliberately avoid finer time-resolution because the published estimates have wide confidence intervals.
How robust is the signal
This is the smallest factor in our model with the most uncertainty. Mikkelsen and colleagues’ large Danish cohort did not find a clinically meaningful difference in time-to-pregnancy by recency of contraception cessation [1] — and twin probability is only one signal in that broader fertility picture. Different studies use different control groups, different time windows and different populations.
Our position: the signal is real but modest, and it decays quickly. We include it because users can verify it (you remember when you stopped the pill) and it interacts informatively with age and family history. We do not believe it justifies any clinical decision on its own.
What this is not
This factor does not apply to non-hormonal methods (copper IUD, condoms, withdrawal). It applies to combined oral contraceptives, progestin-only pills, hormonal IUDs, implants and injectables — any method that suppresses the HPO axis.
If you stopped hormonal contraception more than three months ago, set this factor to “no”. If you stopped within the last three cycles, set it to “yes” and accept that the model is reflecting a small, temporary effect.