Much ink has been spilled, many hands wrung, and money spent over women’s decline in fertility as they age. But what has gone relatively under-reported, and perhaps misunderstood, is that the same is true for men: Sperm quality declines with age, affecting fertility.

After men turn 40, the quality of their sperm’s genetic material lessens; even as their bodies continue to constantly produce new sperm, the new sperm isn’t as good as the sperm they produced when younger. It’s a natural and irreversible effect of aging, much like how women’s eggs deteriorate after age 35. This is true even while the most commonly understood aspects of male fertility — sperm count and sperm motility — may be perfectly fine according to fertility tests.

Which disguises a major factor in fertility struggles, like repeat miscarriage: Declining sperm quality may not impact men’s ability to ejaculate or successfully fertilize an egg, but it is more likely to make an embryo unviable.

Miscarriage is the body’s way of dealing with a pregnancy that can’t proceed further due to genetic abnormalities. Since miscarriage occurs only in women, it is often assumed, or secretly felt, that when it does, there is something wrong with the mother. Yet, it’s just as likely the male contribution is responsible; when one half of the genetic material is degraded from the start — as in the case of men over 40 — it makes it more difficult for an egg to fertilize in a way that’s viable. The result is often an embryo that’s not chromosomally sound enough to develop into a fetus and baby — thus, miscarriage.

That’s not to assign blame to miscarriage — whose DNA is responsible is impossible to parse, and sometimes, even healthy DNA comes together in a way that just doesn’t work. And it’s not to say a couple in which the man is over 40 (or the woman is over 35, or both) cannot successfully conceive a healthy embryo. It’s just that the odds are turning against them. The likelihood of this kind of chromosomal flop — and resulting miscarriage — increases with each passing year, as older men’s sperm contain increasingly incomplete genetic material. Even when a viable embryo is established, it is more likely to develop into a fetus that carries chromosomal abnormalities; father’s age at conception has been linked to increased risk of autism, Down syndrome, schizophrenia and other genetic disorders among children.

So while a man over 40 may be highly fertile on paper (that is, he may not experience difficulty ejaculating or a low sperm count), his age — not just his partner’s — may still be a factor in fertility problems.