1. If fertilization does occur in the uterine tube, the fertilized egg travels to the uterus and implants in the uterin endometrium.

2. The placenta, a vascular organ, now forms within the uterine wall.

3. The amnion is the innermost of the embryonic membranes, and it holds the fetus suspended in an amniotic cavity sorrounded by a fluid called the amniotic fluid. When this breaks, it signals the onset of labor.

4. The uterus normally lies in the pelvis. During pregnacy, the uterus expands as the fetus grows.

5. Progesterone maintains the development of the placenta. Low levels of progesterone can lead to spontaneous abortion in pregnant women and menstrual irregularities in non-pregnant women.

HCG - HUMAN CHORIONIC GONADOTROPIN

The placenta, also known as the afterbirth because it separates from the uterus after delivery, produces its own hormone as it develops in the uterus. This placental hormone is called human chorionic gonadotropin (HCG).

HCG is the hormone tested for in the urine of women who suspect that they are pregnant. HCG stimulates the corpus luteum to continue producing hormones until about the 3rd month of pregnancy, when the placenta itself takes over the endocrine function and releases estrogen and progesterone.