The cover story of this month’s New Republic tells us what we can see with our own eyes—that people are waiting longer to have children. The average age of first-time mothers has increased by about four years since 1970, from 21.5 years to 25.4 and most American men are waiting to become fathers until roughly age 28 (as opposed to about 23 in the 1970s). What is surprising is how big an impact older parenthood will have on the next generation’s genetic makeup.

Judith Shulevitz starts with her own experience trying to have her first child at age 37. She and her husband (in his mid forties) worked with a fertility specialist, finally conceiving after a minor surgery and the fertility drug Clomid. Her little boy seemed perfectly healthy until her pediatrician noticed fine motor delays.  She took her son to a physical therapist, where she met more mothers, many of whom were older. She decided to look into it. What Shulevitz’s research found is that both men and women need to think more carefully at their fertility age. Those who decide in their mid-thirties or later to have children often need to look to fertility treatments. A May article from the New England Journal of Medicine found that children born with the help of fertility treatments and drugs had an 8.3 percent rate of defects whereas babies born without them had a rate of only 5.8 percent.

It’s common knowledge that an infant’s risk of disability dramatically increases once mom is 35 and older, but the article also points to a study from earlier this year that linked the father’s age to the 78 percent rise in autism over the last ten years. Researchers in Iceland have developed a new way of studying genomes and found that as men age, they’re more likely to pass on de novo (spontaneous, non-inherited defects) to their children. Add that to the risk of error as sperm cells divide and other molecules from the environment attach to cause problems, and it’s easy to see why parental age is a serious consideration. Because while women are born with all the eggs they’ll ever have, men begin producing sperm during puberty and continue to produce sperm throughout their lives. Each time a sperm cell splits to create a new one, there’s a greater chance of the genetic material copying incorrectly. This is maybe the biggest, most complicated part of older parenthood:

“Sociologists have devoted many man-hours to demonstrating that older parents are richer, smarter, and more loving, on the whole, than younger ones. And yet the tragic irony of epigenetics is that the same wised-up, more mature parents have had longer to absorb air-borne pollution, endocrine disruptors, pesticides, and herbicides.”

So while more and more people are putting off starting their families to get their relationships and careers in order, it comes with a price: a higher risk of health issues like Down syndrome and cerebral palsy, and figuring out how to care for those children when the parents themselves may need more care because of advanced age. Even if people with children do live longer there’s still the possibility they won’t live long enough or be physically able to provide the care their needs.

Words: Desiree Browne