Menstruation, Ovulation, and Fertility
The menstrual cycle is an average of 28 days, although it is normal to vary between 23 and 35 days. The cycle is counted from the first day of your period. Ovulation is the release of the egg from the ovary. This usually occurs at about day 14 of the cycle. Once released, the egg is fertile for up to 48 hours. If the egg is not fertilized, then the lining of the uterus will disintegrate, causing a period. By knowing your menstrual cycle, it is possible to approximate when you will be most fertile.
The Menstrual Cycle
- The cycle begins on the first day of menstruation, when the lining of the uterus (also called the endometrium) is shed as menstrual blood. This is caused by a decline in the hormones estrogen and progesterone, which occurs when the egg from the previous cycle is not fertilized.
- The decrease in estrogen and progesterone causes the pituitary to release follicle stimulating hormone (FSH). The follicles are the structures inside the ovaries that produce the eggs. Each month, one follicle matures to release the egg for that cycle.
- FSH is produced during days 1-13 of the cycle, causing the follicle to produce estrogen. Estrogen causes the endometrium to grow and thicken in preparation for the fertilized egg. The mucus produced by the cervix (usually noticed as vaginal secretions) becomes thin, clear, and watery.
- Rising levels of estrogen cause the pituitary to release luteinizing hormone (LH), which stimulates the follicle to release the egg into the fallopian tube (oviduct). This is called ovulation, and it usually occurs on day 14 of the menstrual cycle.
- After ovulation, the follicle turns into a structure called the corpus luteum, which produces progesterone, causing the lining of the uterus to thicken more in preparation for the fertilized egg.
- If the egg is not fertilized, the production of estrogen and progesterone drops sharply, which triggers the shedding of the endometrium, and the next period.
- If the egg is fertilized, it will usually implant in the uterus within 3-4 days after ovulation.
You will be most fertile around 2 weeks after the beginning of your last period. This period of fertility will last for about 2 or 3 days afterwards. It is important to remember that sperm can live inside your body for up to 5 days. This means that if you have unprotected sex 5 days before your most fertile day, you can still become pregnant.