A couple of weeks ago, I wrote a piece about the paternal age effect, a fascinating phenomenon that has emerged out of large-scale epidemiological studies linking older fathers with neurological conditions such as autism and schizophrenia in their kids. The basis for the effect is an increased rate of errors creeping into the sperm production line the longer a man waits to have kids. Most of these errors — or de novo mutations — will be harmless, but sometimes, de novo mutations happen to fall within a gene or regulatory sequence necessary for healthy brain function.
The paternal age effect has brought some perspective to the bad press that is usually focused on women who have kids at an older age. This is perhaps because it has been known for far longer that older women have an increased risk of giving birth to babies with chromosomal abnormalities, such as trisomy 21, known more commonly as Down syndrome.
The reason that men pass on de novo mutations, whereas women pass on chromosomal abnormalities is all down to how — and when — sperm and eggs are made.
For women, a dozen or so immature egg precursor cells, called primary oocytes, answer the hormonal call of follicle stimulating hormone (FSH) each menstrual cycle and start their developmental journey to become a fully mature egg. Only one will complete the journey to mature ovum, ready to be released from the ovary for fertilisation.
During maturation, the egg will have gone through two rounds of cell division, and it is in these steps that errors leading to chromosomal abnormalities can occur. Instead of neatly separating the chromosomes into equal consignments at each division, an event called nondisjunction can occur. During nondisjunction, chromosomes are unequally separated, and the result is that some eggs contain too many chromosomes, while others have too few. The older a woman is, the more likely that nondisjunction will occur during egg maturation.
Older women also face other complications during pregnancy, such as increased risk of high blood pressure, gestational diabetes and stillbirth. But as I explain in this piece on ABC Health & Wellbeing, these risks need put into perspective. The overall risks are low, and health factors — such as whether a woman is overweight, is from an ethnic background particularly susceptible to diabetes, or has already had a tribe of children when she gets pregnant post-35 — can have a greater impact on pregnancy outcome than her age alone. There’s also evidence that a man’s obesity can increase his partner’s chances of poor pregnancy outcomes.