While there’s nothing magical that happens when a woman turns 35 that suddenly causes her to be at “high risk”, this age was chosen as a cut-off because the risk of having a child with a chromosomal disorder at age 35 was higher than the risk for miscarriage associated with an amniocentesis (an invasive procedure available in the second trimester which is able to diagnose chromosomal disorders in the fetus). Age 35 was used to designate which women should be offered an amniocentesis during pregnancy. However, times have changed; it’s now a standard-of-care to offer an amniocentesis as well as other screening and diagnostic testing options to all women at any age.
Any woman has a small chance to conceive a baby with a chromosome problem but as we age, the risk increases slightly every year due to the way women’s eggs are made, stored, and ovulated. All of a woman’s eggs begin maturing when she is still a fetus and are then suspended until puberty. After puberty, one egg is matured, divided, and released during each ovulation cycle. As a woman ages, her eggs have been held in suspension for a greater time period increasing the chance that when the egg divides, the genetic material inside the egg (chromosomes) will not divide correctly. If that egg is then fertilized, the embryo may have an extra or missing chromosome, leading to Down syndrome, Trisomy 18, Trisomy 13, or other problems that often lead to miscarriage. For a woman who is 35 on her due date, the chance for her to have a baby with a chromosome problem is about 0.5%. For a woman who is 40, this risk increases to about 1.6%.
If you’d like more information about prenatal genetic screening and testing or to schedule an appointment, please contact Maternal-Fetal Medicine at Intermountain Medical Center or LDS Hospital by calling (801) 507-7400.