The menstrual cycle is the process of preparing the human female body for potential pregnancy. The cycle can be divided into two stages—the ovarian cycle and the uterine cycle. The uterine cycle has three phases that are called menstruation, the proliferative phase, and the secretory phase.

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.

Prolactin is a hormone responsible for the production of milk in the mammary glands of mammals. It is not involved in the menstrual cycle, which is controlled by fluctuating levels of the hormones estrogen, progesterone, luteinizing hormone (LH), and follicle-stimulating hormone (FSH).

Pregnancy related medical conditions and complications are incredibly common due to strain on the mother and extreme biological changes associated with pregnancy. Examples include postpartum depression, anemia, preeclampsia, and ectopic pregnancy.

The release of the secondary oocyte from the follicle is called ovulation. During this process, a hole called the “stigma” is formed and it allows the secondary oocyte to leave the follicle surrounded by a layer of cells called the cumulus oophorus. After its release, the secondary oocyte enters the fallopian tube.

After ejaculation into the female vagina, capacitation of sperm cells takes place. During this process, sperm undergo molecular and biochemical changes that allow them to fertilize the oocyte. In this final stage of maturation, sperm cells lose many membrane proteins (including glycoproteins and steroids to ease binding to the oocyte), undergo an influx of intracellular calcium, and change their tail movement pattern. The changes that occur during sperm activation put the sperm cells into a state of hyperactivity, or heightened motility that aids fertilization.

After fertilization and sperm cell penetration of the oocyte, the cortical reaction takes place. During the cortical reaction, a release of intracellular calcium ions triggers the exocytosis of cortical granules. Cortical granules are vesicles in the cortex of the oocyte that contain enzymes that prevent polyspermy. Exocytosis of the cortical granules releases their contents into the extracellular matrix—the zona pellucida in mammals—creating changes to prevent further sperm penetration.

“Menarche” is the term that refers to the onset of menstrual bleeding in female humans. The timing of menarche is influenced by variety of factors including genetics and environmental factors. Age of menarche is typically between 12 to 15 years old.

During the follicular phase of the menstrual cycle, follicle-stimulating hormone (FSH) stimulates the development of multiple follicles. The follicles compete with each other and the most developed follicle becomes the dominant follicle. The dominant follicle will release an oocyte during ovulation.

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.