How much do you know about human reproduction? Whether you are trying to get pregnant soon or planning to wait for the future, there are some reproductive facts important for everyone to know. Here are a few facts from the physicians at Pacific Fertility Center.

Anatomy

At the middle of the menstrual cycle, an ovary releases (ovulates) a mature egg, which the fallopian tube picks up—one end of the tube is right next to the ovary. The tube moves the egg back towards the uterus.

If sexual intercourse occurs around this time, the sperm swim through the lower opening to the uterus (cervix). They travel though the uterine cavity to the tubal openings and into each tube. Fertilization occurs in the outer third of the tube containing the egg.

Fertilized or not, the egg reaches the uterus about 4–5 days after ovulation. If healthy, a fertilized egg now contains 100–200 cells (and is now called a blastocyst). It imbeds into the uterine lining (endometrium).

If fertilization does not occur or the embryo is not viable, the uterine lining sheds about one week later (the menstrual period).

Hormones and ovulation

Hormones from the pituitary gland at the base of the brain control ovulation. Follicle-stimulating hormone (FSH) prompts an egg to grow and mature. Ovulation does not occur in alternating ovaries in alternating months. Usually, just one egg matures at a time, randomly in either ovary.

A cycle is from the start of one menstrual period to the next. When the egg is mature at the middle of the cycle, the pituitary gland releases luteinizing hormone (LH), called the "LH surge." This prompts the egg to enter into final maturation and to be released. The actual release of the egg is what we refer to as ovulation.

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Timing of intercourse

About 40 hours after the LH surge, the ovary releases an egg. It will need to encounter sperm within about 12 hours to successfully be fertilized. Sperm can live in the female reproductive tract for several days. Therefore, intercourse that occurs anywhere from 3 days before and up to about 12 hours after ovulation can result in fertilization.

However, if a woman is trying to conceive, we recommend intercourse on the day of the LH surge as well as the next day. You can detect LH surges with over-the-counter (OTC) ovulation predictor kits that test the urine for the presence of LH hormone.

Eggs: A limited resource

Women are born with millions of eggs. For unknown reasons, most of these eggs die before they ever have a chance to mature and ovulate. About 1,000 eggs die for every egg that matures and is released.

In reproductive-age women, about 5–25 eggs are present in each cycle. These "finalists" can respond to FSH and mature. As it turns out, however, humans have natural mechanisms that keep all but one of these finalists from growing, which means only one egg will be ovulated, except in the case of fraternal twins and triplets.

Women typically lose the ability to conceive in their early 40s, about 5–10 years before ovulation and menstruation finally ends. Egg freezing is a viable option to preserve eggs when you are younger for use later in life.