Dizygotic twinning is based on a precise mechanism: polyovulation. Understanding the heredity of twins requires a clear distinction between maternal genetic pathways, which are the only ones demonstrated in the transmission of the risk of twin pregnancies, and paternal contribution, which is often overestimated in public discourse.
Hyperovulation Genes: The Molecular Foundation of Dizygotic Twinning
Six genes are currently identified as involved in hyperovulation: FSHB, SMAD3, GNRH1, FSHR, ZFPM1, and IPO8. Each plays a role at a different point in the hormonal cascade that regulates follicular maturation and the release of oocytes.
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FSHB encodes the beta subunit of FSH, the follicle-stimulating hormone. A variant of this gene leads to higher secretion of FSH, promoting the simultaneous maturation of multiple follicles during the same cycle. SMAD3 acts downstream, in intracellular signaling within ovarian granulosa cells.
FSHR, the gene for the FSH receptor, modifies ovarian sensitivity to this hormone. A woman carrying a high-response variant can produce two mature oocytes even with normal FSH levels. Here we see that polyovulation results from multigenic interactions, not from a single gene.
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GNRH1 is involved upstream, at the hypothalamic level, while ZFPM1 and IPO8 participate in the transcriptional regulation and nuclear transport of ovarian growth factors. This complex network explains why the probability of dizygotic twins varies so much from one family to another and from one population to another. A detailed article on the heredity of twins from the father and mother specifies the transmission mechanisms unique to each parental lineage.
Maternal Transmission of Twin Risk: Why the Father Does Not Transmit Polyovulation
Only the mother can express hyperovulation genes, because these genes act on ovarian physiology. A man can carry the same genetic variants without them having any effect on his own fertility: he does not have ovaries.
The common confusion arises from the fact that a father can pass on a hyperovulation allele to his daughter. If this daughter inherits it, she will have an increased risk of dizygotic twin pregnancy herself. Thus, the father plays a role as a silent vector across one generation, not as a direct trigger.
This generational shift creates a typical pattern in population genetics: a woman with dizygotic twins often has a mother, sister, or maternal aunt in the same situation. The paternal link skips a generation and passes through the daughters of the carrier father.
Practical Consequence in Genetic Counseling
When a couple asks a doctor about their risk of twin pregnancy, the maternal family history on the woman’s side is the most informative. The paternal history only becomes relevant if the father has sisters or daughters who have had dizygotic twins, indicating that he likely carries a transmissible hyperovulation variant.
Monozygotic Twinning: A Phenomenon Without Demonstrated Hereditary Component
Monozygotic twins result from the division of a single embryo after fertilization. The frequency of this division remains stable at around 0.4% of pregnancies, regardless of ethnic origin, country, or family history. No parental gene has been identified as modulating this risk.
This global stability sharply contrasts with the geographical variability of dizygotic twin rates. West African countries show significantly higher twin birth rates than the global average, almost exclusively driven by dizygotic twins. This differential reflects the higher frequency of hyperovulation variants in these populations.
Monozygotic twinning is still considered a random event. Current hypotheses suggest mechanical or biochemical disturbances at the blastocyst stage, but no reproducible familial transmission has been demonstrated to date.
Non-Genetic Factors That Modify the Risk of Dizygotic Twins
Genetics is not the only lever. Several physiological and environmental factors increase the likelihood of superovulation independently of genetic heritage:
- Maternal age beyond 35 years leads to a natural elevation of FSH, stimulating the maturation of multiple follicles per cycle. This mechanism compensates for the decline in ovarian reserve and increases the risk of dizygotic twin pregnancy.
- A high body mass index is associated with higher levels of estrogen and insulin, two hormones that promote multiple follicular recruitment.
- Fertility treatments, particularly ovarian stimulation protocols, represent the most powerful exogenous factor. Clomiphene citrate and injectable gonadotropins significantly increase the number of mature follicles.
- Parity also plays a role: women who have already had multiple pregnancies have a slightly higher twin risk, likely related to cumulative hormonal changes.
The number of twin births worldwide has increased by 30% since the 1980s, according to a study published in Human Reproduction. The rise of medically assisted reproduction explains a major part of this increase, more than the genetic evolution of populations over such a short period.
Geographical Variability of Twin Rates: A Population Genetic Marker
The global distribution of dizygotic twin pregnancies serves as an indicator of the frequency of hyperovulation alleles. West African populations have the highest rates, while East Asian populations show the lowest rates. Europe falls within an intermediate zone.
This distribution cannot be explained solely by access to healthcare or environmental factors. It reflects genetic selection over thousands of years, with variable evolutionary pressure across regions. The frequency of FSHB and SMAD3 variants follows this same geographical gradient.
The recent increase in twin rates in high-income countries, however, is linked to assisted reproductive technology and the delay in the age of first pregnancy, two non-genetic factors that overlap with hereditary substrate. Distinguishing the genetic contribution from the environmental contribution remains an active challenge in reproductive epidemiology.
The distinction between dizygotic twinning (heritable, maternal, polygenic) and monozygotic (random, stable, non-heritable) structures the entire clinical and genetic approach to the subject. A couple wishing to assess their risk of twin pregnancy benefits from focusing on the maternal family history of the woman and on modifiable hormonal factors, rather than on the direct paternal lineage.