How does age affect fertility?

It can be concluded that age affects a woman's fertility in two ways; by reducing the number of eggs and their quality, and by changing hormone levels, which affects your menstrual cycle.

Your egg reserve and egg quality decrease

You are born with millions of eggs, but these gradually disappear over the years, and when you reach puberty and begin to ovulate and menstruate (and become fertile), you have only around 400,000 ovaries left. It is often said that when you are around 30, you have about 12% of your original egg reserve left and at 40 the percentage has dropped to 3%.

This might sound scary and there is often a high focus on the number of eggs when talking about fertility, but staring blindly at your exact egg reserve is the wrong focus. The number of eggs is just one of the things that matter and, to get pregnant, you only need one healthy egg – it is more common that you have problems ovulating than that you have too few eggs. With that said, of course, a very low egg reserve means that the odds per menstrual cycle decrease. Especially considering that not all eggs can become a healthy baby. For an egg to be able to lead to a healthy pregnancy, it needs to have the right quality, meaning it has a normal number of chromosomes.

Just like all other cells in our body, the egg cells age and the proportion of healthy eggs decreases over time. Just like our skin becomes wrinkled with age, the egg quality gets poorer. If the egg or sperm does not have the right number of chromosomes, then either no fertilization takes place at all, or you get an early miscarriage. Not only does it become more difficult to get pregnant with age, but the risk of miscarriage also increases.

When you are around 25 years old, about 75% of the egg systems have a normal set of chromosomes. When you reach 35 years the proportion is 50% and at 40 years it has decreased to 10-15% – on average.

You can get an indication of your egg reserve by measuring AMH and by ultrasound. Egg quality is more difficult to measure proactively. With an IVF you can get more insights, but you would not do an IVF just to measure egg quality. So when we talk about proactivity, age is the best indicator.

Your hormone levels change along with your ovulation and menstrual cycle

The very last period, the end of a woman's fertile life, is called menopause and occurs on average at 51 years. The phase that leads up to menopause can start from a few years before menopause to more than ten years before and involves changes in hormone balance and the cycle.

This also has to do with the eggs, because as the number of follicles in your ovaries decreases, so does for example the amount of estrogen, as it is produced by the follicles. This in turn affects other hormone levels. When the pituitary gland in your brain senses that estrogen and other hormone levels are lowering, it tries to compensate by instead increasing the production of follicle-stimulating hormone (FSH), especially in the early follicular phase of your menstrual cycle. An FSH test that in that phase shows lowered values can therefore be an indicator that you are approaching menopause.

During these changes in hormone levels you can experience symptoms such as spottings between ovulation and your next period, and changes in the length of the menstrual cycle – usually the cycle first becomes shorter and then longer. This does not mean that you are not fertile at all, but it is a sign that it gets harder and harder for the body to ovulate (hence the irregular cycle), and ovulation is crucial to be able to get pregnant. A changed ovulation pattern is thus another way in which age affects fertility.

2022-03-31

Evangelia Elenis, MD, PhD.

This text is fact checked by Evangelia Elenis, MD, PhD. Dr. Elenis is a chief physician in Obstetrics and Gynecology, and a subspecialist in Reproductive Medicine. She is a PhD and affiliated researcher at Uppsala University with postdoctoral studies at Harvard Medical School.

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