Ovulation occurs when a mature egg is released from the ovary. It travels through the fallopian tube and is either fertilized with sperm to implant in the uterus for pregnancy or disintegrates before the uterine lining is flushed out during the next menstrual cycle. Ovulation usually happens at day 14 of a 28-day cycle, but because every woman’s cycle is a bit different the exact timing can vary.
In the days leading up to ovulation, your body will experience an increase of luteinizing hormone (LH). This triggers the egg’s release, typically about 36 hours after the surge of LH. While a woman typically ovulates one egg each month, occasionally two will be released. If they are fertilized, the result is fraternal twins.
Incredibly, a female fetus has about 6 to 7 million eggs. At birth, the number is closer to 1 million, and she is born with all the eggs she will ever have. By puberty, she will have about 300,000, though, according to Cleveland Clinic, only 300 to 400 of those eggs will actually be ovulated. From puberty, the number of available eggs declines steadily. She may lose up to 1,000 eggs each month, meaning that at age 25 there will be 200,000 available eggs; by her mid-30s, the number drops even more drastically. This chart, sourced from the American Society for Reproductive Medicine (ASRM), shows egg count statistics through the age of 50.
From a biological perspective, science tells us that the best time for a woman to have children is in her late teens to early twenties. The reproductive system and other functions are at their peak, and eggs are still fresh. The reality, though, is that many women choose to wait. The risk in delaying pregnancy is diminished ovarian reserve: ASRM reports that a 30-year-old woman has about a 20 percent chance of conceiving each month; a 40-year-old’s chances dip closer to 5 percent. Between fewer available eggs and a higher risk for genetic abnormalities, it can be more difficult for a woman to have a successful pregnancy beyond the age of 35. A woman’s biological clock begins to speed up around this time; it’s recommended that a woman over 35 see a reproductive endocrinologist if she’s unsuccessful in conceiving after 6 months, while a woman under 35 should have a workup after one year.
While your age may give some indication, there are several more concrete ways to determine if you have enough eggs to conceive. Work with a reliable reproductive endocrinologist to have hormone testing and an ultrasound. They will take a look at your follicle-stimulating hormone (FSH) level, which regulates the function of the ovaries, and your anti-mullerian hormone (AMH) level, which can reflect your ovarian reserve. An ultrasound to count antral follicles is also recommended.
Together, these results will give your doctor a better idea about next steps; you may get the go-ahead to continue trying to conceive naturally, or if you have diminished ovarian reserve, you may begin to explore assisted reproductive technology (ART). Depending on the situation, your doctor may recommend IVF with your eggs or with the assistance of an egg donor.
It should be noted that it’s not just the number of eggs that is important, it’s also the quality. A diminishing ovarian reserve coupled with genetic issues can make it more difficult for a woman to become pregnant. As eggs age, the chances of genetic abnormalities also increase.
21-28 years old is the recommended age range for egg donors, though the range is significantly larger for a woman doing IVF with her own eggs. Your clinic may attempt egg retrieval into your early 40s, some close to 50. In an attempt to circumvent diminished ovarian reserve and age-related genetic abnormalities, many women in their 20s and 30s are considering egg freezing. In this relatively new procedure, eggs are retrieved and cryopreserved so that, in theory, they can eventually be thawed and fertilized with sperm for an IVF transfer even years down the line.