Risk of Aneuploidy and maternal age
As a woman ages, the chance of having an aneuploid pregnancy
increases. This association is because a woman?s eggs are as
old as she. Females have all of their eggs in the fetal
stage therefore they are born with all the eggs they will
have in their lifetime. In males, sperm is made every 65-75
days therefore the sperm is not as old as the man.
Therefore, the theory regarding aneuploidy risk and
advancing maternal age is that over time the chromosomes in
the egg are less likely to divide properly leading to the
egg having an extra or missing chromosome. There is a
significant rate of chromosome abnormalities in embryos, and
the frequency of these abnormalities increases with age.
Only a few pregnancies carrying an extra or missing
chromosome will go to term; most will be miscarried.
The likelihood of delivering an affected child is shown in
the chart below*.
Maternal Age at Delivery
Risk of Aneuploidy in
||1 in 385 (or 0.26%)
||1 in 192 (or 0.52%)
||1 in 102 (or 0.98%
||1 in 66 (or 1.5%)
||1 in 21 (or 4.8%)
||1 in 8 (or 12.5%)
*Hook EB. Cross PK. Schreinemachers DM. (1983) Chromosomal
abnormality rates at amniocentesis and in live-born infants.
JAMA, 249(15):2034-8. *Hook EB. (1981) Rates of chromosomal
abnormalities at different maternal ages, Obstetrics &
Table: Women of advanced
maternal age are at increased risk of producing an embryo
affected with a genetic disease.
|15 - 19
|20 - 24
|25 - 29
|30 - 34
|35 - 39
|40 - 44
However, the frequency of aneuploidy in embryos is much
higher than what would be expected looking only at affected
live borns. For women between the ages of 35 to 39 years
approximately 40% to 50% embryos are abnormal. For women 40
years and older, on average, greater than 50% of embryos are
difference in percentages in embryos versus live born is due
to the fact that a pregnancy with aneuploidy is less likely
to implant to the uterus or go to term. Most will be
miscarried. As such, the percentage of affected pregnancies
is reduced over the course of the pregnancy due to the
affected pregnancies that are lost. Any embryo with a
missing chromosome (monosomy) will cease to grow before
implantation (except monosomy X and 21), and only few of
those carrying an extra chromosome (trisomy) will go to
term. The lack of implantation and loss rate of aneuploid
embryos are believed to be the main reasons why the
pregnancy rate in women over 40 is so low.
The goal of preimplantation genetic diagnosis for
chromosomal aneuploidy is to select for and maximize the
chance of transferring only chromosomally normal embryos to
achieve more pregnancies, reduce the number of pregnancy
losses, and reduce the number of affected offspring.