The association between fetal chromosomal abnormalities and older maternal age has been widely researched and established. The older a woman decides to have a child, the greater the chances of her baby being conceived with a chromosomal defect such as Down’s Syndrome. This is because a woman’s eggs age as she ages. Older eggs are more prone to forming embryos with either too many or too few chromosomes. This is the reason why older women have a greater rate for infertility, miscarriages and babies with chromosomal birth defects. Many embryonic chromosomal abnormalities in women undergoing IVF can be detected using PGD or preimplantation genetic diagnosis.
Age and risk for non-chromosomal abnormalities
Now, however, it has come to the attention of researchers that the extremes of maternal age, meaning women over age 35 and women who are 20 and under, may also be related to non-chromosomal structural abnormalities in the fetus.
The average age of women giving birth has increased over the last few decades from 24.6 years to 27.2 years. To go along with this, birth rates have risen in women age 30-34 and 35-39. For moms ages 35-39, the prevalence of birth defects ranges from 32-44 per 1000 births. When compared with moms aged 25-29, there is an extra 5 to 12 non-chromosomal birth defects for every 1000 births. In moms 40 years and older 24-50 per 1000 births are affected by these non-chromosomal abnormalities. This is an increase of about 6-11 birth defects for every 1000 births.
Some specific abnormalities found to be associated with advanced maternal age include congenital heart defects, hypospadias (abnormal male urethra), craniosynostosis (abnormal skull growth), club foot, and diaphragmatic hernia. Women ages 35-39 will deliver 1 to 4 additional cases of congenital heart defects per 1000 births. In age 40 and over, that number jumps to 30.
As with older mothers, when considering the overall risk of structural birth defects, there is a significant association with increased odds of birth defects and maternal age less than 20 years of age. In mothers younger than 15 years old, the prevalence of these abnormalities is anywhere from 37 to 46.9 per 1000 births. This translates to an extra 27 birth defects per 1000 births when compared with mothers aged 25-29 years old who have the lowest risk.
Specific abnormalities found associated with younger mothers are anencephaly- born without a forebrain, hydrocephaly- accumulation of fluid in the brain, ear defects, cleft lip, female genital defects, polydactyly- too many fingers and toes, omphalocele or gastrochisis- abdominal defects that leave organs outside the body.
In mothers 20 and younger, gastrochisis is the most common birth defect. The prevalence in this age group is 7 to 26 per 10,000 births in women aged 20 and younger. This is the largest of any defect by far.
It is well known that the use of prenatal testing in older women is more common. These tests could lead to more frequent diagnosis of abnormalities and possibly more early pregnancy terminations, reducing the number of defects seen at birth.
Apart from theses errors, there are potential mechanisms that could account for the real increase of defects in younger and older mothers. Multivitamin use is lowest among younger pregnant women. Exposure to smoking, alcohol, and drugs also vary according to age. Further research is needed to definitively reach conclusions about what causes these differences and how they can be avoided.
Detection of birth defects
About 65% of women who are pregnant in the U.S. undergo an ultrasound during their pregnancy. Depending on the age of the fetus at the time of the scan, the detection rate of birth defects varies ranging from 16% to 85%. Generally, screening the higher risk population is more effective than screening the entire population. However, because of the low general risk, screening moms younger than 20 may not be cost effective. This issue, however, has not been specifically addressed.
While biological explanations for this increased health risk are unclear, it must be noted that although the risk of having a baby with birth defects is higher for younger and older women, the overall absolute increase in the number of babies with birth defects is low. In addition, because birth defects are rare, the number of women giving birth at the extremes of age is significantly less than the number of women aged 20 to 35 years. Nevertheless, these findings are significant and should be carefully considered when diagnosing congenital defects.