Low Birth Weight in Men Is Linked to Infertility
Men born below the 10th percentile for birth weight had up to a 55% increased risk of male-factor infertility later in life in a Danish study -- but the same pattern wasn't found for women's own birth weight and fertility.
You might be thinking: another weight-and-infertility post. But this time it's not about current weight — it's about how much your male partner weighed the day he was born.
Why researchers looked at this
With about 50% of infertility cases involving a male factor, in part or in whole, it matters that fertility research doesn't focus on women alone. This particular study looked at infertility rates in both men and women, with an eye on early-life factors.
Previous research had already found differences in hormone levels and testicular volume between low-birth-weight and average-birth-weight men, but it wasn't clear whether that translated into actual infertility later in life. Researchers from Denmark set out to find out.
What they found
Men born very small for gestational age (birth weight below the 10th percentile) had up to a 55% increased risk of male-factor infertility later in life, compared to men with a higher birth weight. Part of that elevated risk can be explained by health conditions associated with premature birth that are also independently linked to infertility. Interestingly, the same pattern didn't hold for women: having a very low birth weight didn't appear to matter for their own fertility.
Why this matters beyond the statistic
Male fertility factors are often under-discussed compared to female ones, even though they account for roughly half of all infertility cases. Birth weight isn't something anyone can change after the fact, but it's a useful reminder that male-factor infertility has real biological roots worth investigating — not just a female partner's cycle, age, or hormones. If a couple is struggling to conceive, a full fertility workup for both partners (rather than assuming the issue lies with the woman) is worth pursuing early rather than late.
Common questions
Does this mean my male partner is infertile if he was a small baby?
No -- it's an increased relative risk across a large population, not a diagnosis for any individual. Most men born small for gestational age go on to have entirely normal fertility.
Why does male birth weight matter for fertility research specifically?
About half of infertility cases involve a male-factor component, in part or in whole, so male-side risk factors deserve just as much research attention as female-side ones -- historically, they've gotten a lot less.