Combination birth control pills contain estrogen and progesterone. Supplementing progesterone levels inhibit luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion, follicle development, and ovulation. Estrogen supplementation decreases FSH, thereby inhibiting follicle development and ovulation. The combination pill decreases breakthrough bleeding but has more withdrawal symptoms.

Common methods of fertility testing include: calendar tracking, ovulation predictor kits, and monitory physical symptoms. Calendar tracking determines fertility by cycle length. Ovulation predictor kits measure luteinizing hormone (LH) levels in an individual’s urine. Monitoring physical symptoms such as cervical mucus and basal body temperature are also a good method of tracking fertility.

During the menstrual cycle, the time when fertility peaks is at ovulation and a few days preceding it. At this point, sperm can survive in the female reproductive tract until oocyte release.

Amenorrhea is a condition where menstrual cycles are irregular and rare, occurring every 3 to 6 months.

Menstrual disorders include a heavy flow (menorrhagia), irregular cycles (amenorrhea, polymenorrhea, metrorhagia, and oligomenorrhea), and no ovulation (anovulation).

There are variations in the age of menarche by geographic location and environmental conditions. In developing countries, the age of menarche is later than in industrialized nations.

The endometrium is a mucus membrane in the uterus of the female reproductive tract. The endometrium changes thickness based on hormone levels and is shed during the menstrual cycle if fertilization does not occur.

Human pregnancy can be divided into three trimesters, each categorized by different symptoms and stages of fetal development. During the second trimester of pregnancy, which occurs between 13 and 28 weeks, fetal movement can be felt. This is often called “quickening,” which is the stirring of the fetus.

The third trimester of pregnancy takes place during weeks 29 and 40. During this stage of pregnancy, women continue to gain weight, the fetus moves regularly, and the fetal head descends. The descent of the fetal head is important in easing maternal breathing and positioning the fetus for future delivery.

Morning sickness is the feeling of nausea and vomiting that occurs in many women during the first trimester of pregnancy. The physiological causes behind morning sickness include low blood sugar, an increased sensitivity to odors, and increased levels of estrogen and gonadotropin-releasing hormone. It is believed that morning sickness evolved in order to prevent the mother from ingesting toxins that could harm the developing fetus. This is supported by the fact that during the time morning sickness occurs, the fetus is highly sensitive to toxins.