Pregnancy and Conception

Most doctors calculate the start of pregnancy from the first day of your last menstrual period. This is called the "menstrual age" and is about two weeks ahead of when conception actually occurs.

Here's a primer on conception:

Ovulation

Each month, in one of a woman's two ovaries, a group of immature eggs start to develop in small fluid-filled cysts called follicles. Normally, one of the follicles is selected to complete development (maturation). This "dominant follicle" suppresses the growth of all of the other follicles, which stop growing and degenerate. The mature follicle ruptures and releases the egg from the ovary (ovulation). Ovulation generally occurs about two weeks before a woman's next menstrual period begins.

Development of Corpus Luteum

After ovulation, the ruptured follicle develops into a structure called the corpus luteum, which secretes two hormones, progesterone and estrogen. The progesterone helps prepare the endometrium (lining of the uterus) for the embryo to implant by thickening it.

Release of Egg

The egg is released and travels into the fallopian tube where it remains until a single sperm penetrates it during fertilization (the union of egg and sperm; see below). The egg can be fertilized for about 24 hours after ovulation. On average, ovulation and fertilization occurs about two weeks after your last menstrual period.

Menses

If no sperm is around to fertilize the egg, it and the corpus luteum will degenerate, removing the high level of hormones. This causes the endometrium to slough off, resulting in menstrual bleeding. Then the cycle repeats itself.

Fertilization

If sperm does meet and penetrate a mature egg after ovulation, it will fertilize it. When the sperm penetrates the egg, changes occur in the protein coating around it to prevent other sperm from entering. At the moment of fertilization, your baby's genetic make-up is complete, including its sex. Since the mother can provide only X chromosomes (she's XX), if a Y sperm fertilizes the egg, your baby will be a boy (XY); if an X sperm fertilizes the egg, your baby will be a girl (XX).

Implantation

Within 24-hours after fertilization, the egg begins dividing rapidly into many cells. It remains in the fallopian tube for about three days. The fertilized egg (called a zygote) continues to divide as it passes slowly through the fallopian tube to the uterus where its next job is to attach to the endometrium (a process called implantation). First the zygote becomes a solid ball of cells, then it becomes a hollow ball of cells called a blastocyst. Before implantation, the blastocyst breaks out of its protective covering. When the blastocyst establishes contact with the endometrium, an exchange of hormones helps the blastocyst attach. Some women notice spotting (or slight bleeding) for one or two days around the time of implantation. The endometrium becomes thicker and the cervix is sealed by a plug of mucus.

Within three weeks, the blastocyst cells begin to grow as clumps of cells within that little ball, and the baby's first nerve cells have already formed. Your developing baby is called an embryo from the moment of conception to the eighth week of pregnancy. After the eighth week and until the moment of birth, your developing baby is called a fetus.