How does age affect fertility?

 

This is an interesting question and addresses an area where much research is being carried out. The adverse effect of age does not appear to be mediated by a decrease in "fertilizability," but rather seems related to abnormal chromosomes in the egg.

Let's go back to review how the egg forms. All human cells other than sperm and egg normally have 46 chromosomes. The egg and sperm each contribute 23 chromosomes to the developing fetus. This means that as the egg and sperm are formed, the number of chromosomes needs to reduced to 23. This process of chromosome reduction is called meiosis.

In men, the process is ongoing, and new sperm are continually being produced. In the woman, the situation is a bit different. Before her birth, while she is still an embryo, the number of her eggs increases up to about 4-7 million. After about 20 weeks of gestation, her fetal body stops producing new eggs. These eggs must also undergo the process of meiosis to reduce from 46 to 23 chromosomes. But at 20 weeks, the eggs are surrounded by an envelope of cumulus cells; this arrests the meiotic process and keeps the eggs healthy until they're needed for ovulation. An egg resumes its growth about three months before ovulation. In fact, meiosis is not actually completed until after ovulation and fertilization has occurred.

We know that older women ovulate eggs that are more likely to contain chromosomal abnormalities, such as extra or missing chromosomes. What we don't know is when this anomaly occurs. Does it occur while the eggs are dormant, in a state of suspended animation awaiting their chance to grow and ovulate? Or does it occur after hormonal signals involved in ovulation stimulate the egg to resume meiosis?

Researchers have hypothesized that perhaps the cumulus cells surrounding the egg lose their ability over time to maintain healthy eggs. Some believe that chemical abnormalities within the cell are responsible for errors in chromosomal reduction (meiosis). At present, no means of restoring egg quality has been identified in women with age related fertility problems.

Fertility gradually declines for women in their thirties and continues to diminish until their fifties, depending on their personal biological schedule. Many women think that the signs of perimenopause mean that they are no longer fertile. In reality, fertility persists until postmenopause. Women in their forties have unplanned pregnancies at a rate second only to that of teenagers. Postmenopausal women cannot become pregnant. Perimenopausal women can and do.

A woman's age can have a significant impact on fertility. That's because her eggs get older as she does. In this video, obstetrician and gynecologist Evelyn Minaya, M.D., discusses other ways in which age affects fertility.

A woman’s age is one of the most important predictors of fertility. Women have approximately 350,000 eggs at the time of puberty, (approximately age 13). Each month, during a woman’s menstrual cycle, dozens of these eggs will begin to develop, but only one will mature and actually ovulate. The remaining eggs will die and not be replaced. As a result, a woman’s overall egg pool diminishes each month. By the age of 30 a typical woman will have lost 88% of her overall egg pool. By age 40 the loss will have reached 97%. Despite these losses, monthly birth rates are well maintained through age 35. For example, every month, the chance of a woman’s egg developing into a pregnancy and ultimately a baby is 20% at age 25, and 15% at age 35. By the age of 40 that rate drops to 10% and by 45 to 2%. The reason for the rapid decline in birth rates beyond the age of 35 is not only due to lower egg counts but also decline in egg quality. Egg quality refers to the egg’s ability to create a chromosomally normal embryo. It is the egg and not the sperm that is responsible for early embryo development. Any mistake at this early stage, especially as it relates to chromosome segregation, will lead to an abnormal embryo that is likely to either die before forming a pregnancy or be miscarried by the 12th week of pregnancy.