Fertilization

A woman's reproductive system includes:

• Two ovaries, which contain the eggs (ova).
• Two fallopian tubes, which connect the ovaries to the uterus.
• The uterus, which contains the developing fetus during pregnancy and contracts during delivery to push the baby out.
• The cervix, which thins (effaces) and opens (dilates) to allow the baby to be pushed out of the uterus.
• The birth canal (vagina).

Humans reproduce through the joining of a male reproductive cell (sperm) with a female reproductive cell (egg or ovum). The sperm and egg each contain half the number of chromosomes required for an embryo to grow and develop. When a sperm fertilizes an egg, the resulting embryo has a complete set of 46 chromosomes.

A women is fertile, or able to become pregnant, during the years that she is having menstrual periods. Approximately once a month, an egg is released from one of her ovaries (ovulation). The date of ovulation varies widely from woman to woman, ranging from day 10 to day 22 of a regular menstrual cycle (day 1 is the first day of menstrual bleeding).¹ Women with irregular cycles have an even greater range of possible ovulation days, but a decreased chance of becoming pregnant.

To become pregnant, a woman must have sexual intercourse close to the time that she ovulates. Sperm can live in a woman's uterus and fallopian tubes for 3 to 5 days, but the egg lives only about 24 hours. Fertilization happens when a sperm cell penetrates the egg, usually in one of the fallopian tubes. The chromosomes carried in the head of the sperm are released and join the chromosomes in the egg to form a complete set. The fertilized egg immediately begins dividing.

See an illustration of fertilization.

While dividing, the egg moves down the fallopian tube to the uterus, which can take several days. By the time it reaches the uterus, the fertilized egg has become a hollow ball of cells (blastocyst). At this point the blastocyst is so small that it can only be seen through a microscope.

Almost as soon as it forms, the blastocyst starts releasing a pregnancy hormone called human chorionic gonadotropin (hCG). This hormone is what pregnancy tests detect in the urine or blood. The release of hCG signals the mother's body to:

• Stop releasing eggs (ovulating).
• Prepare the lining of the uterus for the fertilized egg. If a woman is not pregnant, her body sheds the uterine lining (menstruation, or menstrual period) 11 to 17 days after ovulation. A missed period (lack of menstruation) is one sign of pregnancy.
• Continue producing another hormone called progesterone. Progesterone has many functions in pregnancy, one of which is to prevent the uterus from pushing out the developing fetus.

Citations

1. Wilcox AJ, et al. (2000). The timing of the "fertile window" in the menstrual cycle: Day-specific estimates from a prospective study. BMJ, 321(7271): 1259–1262.