Dizygotic twinning, which results in “fraternal twins,” occurs from the fertilization of two distinct eggs by two different sperm during the same cycle. This double ovulation, known as hyperovulation, depends on hormonal mechanisms specific to the mother. The father does not intervene in this ovarian process, which redefines the actual role of each parent in the probability of a twin pregnancy.
Maternal hyperovulation: the central genetic mechanism
For a dizygotic pregnancy to occur, the maternal organism must release at least two mature oocytes during the same cycle. This phenomenon relies on the regulation of FSH (follicle-stimulating hormone), which drives the maturation of ovarian follicles.
Research published since 2016 has identified polymorphisms in the FSHB and SMAD3 genes, directly linked to FSH production and follicular development. Complementary genomic analyses have then highlighted at least four other genes associated with the conception of dizygotic twins: GNRH1, FSHR, ZFPM1, and IPO8, all involved in the ovarian response to gonadotropins.
All these variants outline a maternal genetic profile of a “super-ovulator”, according to a synthesis published in Sciences et Avenir referencing a study from Human Reproduction. Specifically, a woman carrying several of these variants has an increased probability of releasing two eggs simultaneously, and thus conceiving fraternal twins.
When questioning the twin gene of the father or mother, the answer lies in this mechanism: only the maternal reproductive system decides the number of eggs available for fertilization.
Dizygotic twinning and the father’s role: an indirect influence
The father does not produce eggs. His genetic contribution cannot directly trigger double ovulation in his partner. A man from a family of dizygotic twins does not transmit any hormonal signals capable of altering the mother’s ovarian cycle.
However, his role remains indirect. If he carries genetic variants linked to hyperovulation (inherited from his own mother), he can pass them on to his daughters. These daughters could then, in turn, have a higher probability of conceiving dizygotic twins.
The father acts as a silent vector of maternal hyperovulation genes. This mechanism explains why some families have twins “on the father’s side” without him having played a direct role in the twin pregnancy. The twinning simply skipped a generation, passing from the paternal grandmother to the granddaughter.
Monozygotic twins: neither the father nor the mother has an established genetic role
Monozygotic twins (identical twins) form when a single egg fertilized by a single sperm spontaneously divides into two embryos. This cleavage produces two individuals sharing the same DNA.
In contrast to dizygotic twinning, this phenomenon does not seem to obey any identified hereditary component. The division of the embryo is still considered a random event, unrelated to the family history of either parent.
The distinction between the two types of twinning is therefore fundamental to answering the question of heredity:
- Dizygotic twinning depends on a transmissible maternal genetic background (genes FSHB, SMAD3, GNRH1, FSHR, ZFPM1, IPO8)
- Monozygotic twinning does not respond to any known genetic determinism, neither maternal nor paternal
- The father can transmit hyperovulation variants to his daughters, but without effect on his own partner
Non-genetic factors that modify the rate of twin pregnancies
Maternal genetics does not explain everything. Several environmental and medical factors modify the probability of conceiving dizygotic twins, sometimes significantly.
Maternal age and number of previous pregnancies
The rate of FSH naturally increases with age, which favors the simultaneous maturation of several follicles. Women over 35 have an increased probability of twin pregnancies compared to younger women. The number of previous pregnancies also plays a role: multiparous women statistically have a higher chance of conceiving dizygotic twins.
Fertility treatments and assisted reproductive technology
Ovarian stimulations (clomiphene citrate, gonadotropin injections) increase the number of mature follicles per cycle. In vitro fertilization, when it involves the transfer of multiple embryos, mechanically raises the risk of multiple pregnancies. The number of twin births has increased by about 30% since the 1980s according to a study published in Human Reproduction, and fertility treatments account for a major part of this increase.
- Ovarian stimulation can multiply the number of released oocytes, making it possible to fertilize two or even three eggs
- The transfer of multiple embryos in IVF remains the most direct cause of medically induced multiple pregnancies
- Some countries have modified their protocols to limit the number of transferred embryos, reducing triple and quadruple pregnancies
Heredity of twins: what genetics can really predict
Having twins in the family guarantees nothing. The identified genetic variants increase the probability of double ovulation, but each cycle remains subject to many hormonal and environmental variables.
A woman carrying variants on FSHB and SMAD3 will not necessarily have a twin pregnancy. Conversely, a woman without any known family history may conceive fraternal twins due to age or hormonal treatment.
Genetic predisposition creates a favorable environment, not a certainty. And this predisposition, for dizygotic twins, is transmitted exclusively through the maternal line in terms of direct effect. The father merely relays these genes to the next generation, without impacting the current pregnancy. Identical twins, on the other hand, remain a phenomenon that genetics still cannot anticipate.