Anatomy, Physiology, and Terminology of Reproduction and Pregnancy

Anatomy, Physiology, and Terminology of Reproduction and Pregnancy

Female Reproductive Anatomy

The female reproductive tract consists of several key structures. The ovaries are paired glands that produce oocytes (eggs) and the hormones estrogen and progesterone. Each month during the reproductive years, one follicle matures and releases an egg during ovulation. The fallopian tubes (uterine tubes) extend from the uterus toward each ovary; fertilization normally occurs in the ampulla, the widest segment of the tube.

The uterus is a hollow, muscular organ divided into the fundus (top), body (corpus), and cervix (lower narrow portion). The uterine wall has three layers: the perimetrium (outer serosal layer), myometrium (thick smooth muscle), and endometrium (inner mucous membrane that thickens each cycle and is shed during menstruation). The cervix contains the internal os (opening toward the uterus) and external os (opening toward the vagina). Cervical mucus changes consistency across the cycle, becoming thin and stretchy at ovulation to assist sperm transport.

The vagina is a muscular canal leading from the cervix to the external genitalia. The external female genitalia collectively are called the vulva, which includes the labia majora, labia minora, clitoris, and vaginal vestibule.

Male Reproductive Anatomy

The testes produce spermatozoa and testosterone within the scrotum, where the temperature is slightly lower than core body temperature, which is necessary for sperm production. Sperm mature in the epididymis and travel through the vas deferens to the urethra. The seminal vesicles, prostate gland, and bulbourethral glands add secretions that form semen, providing nutrients and an alkaline environment to protect sperm.

Fertilization and Implantation

Fertilization occurs when one spermatozoon penetrates the oocyte, typically in the fallopian tube within 12 to 24 hours of ovulation. The result is a zygote containing 46 chromosomes (23 from each parent). The zygote undergoes rapid cell division (cleavage) as it travels toward the uterus. By day 3–4 it becomes a morula (solid ball of cells); by day 5–6 it becomes a blastocyst (fluid-filled structure). Implantation into the endometrium occurs approximately 6 to 10 days after fertilization. The outer layer of the blastocyst, the trophoblast, invades the endometrium and begins forming the placenta.

Fetal Development by Trimester

First Trimester (weeks 1–12): This is the period of greatest vulnerability. By the end of week 8, all major organ systems have begun to form; the developing human is called an embryo until the end of week 8, then a fetus from week 9 onward. The heart begins beating around week 6. Neural tube closure occurs by week 4, which is why adequate folic acid before and early in pregnancy is critical. By week 12, the fetus has recognizable facial features and limbs.

Second Trimester (weeks 13–27): Organ systems continue to mature. Fetal movement (quickening) is typically felt by the mother between weeks 16 and 20. Lanugo (fine body hair) and vernix caseosa (protective waxy coating) develop. By week 24, the fetus may reach the threshold of viability with intensive support, though outcomes vary significantly.

Third Trimester (weeks 28–40): Rapid weight gain occurs. The lungs mature, producing surfactant (a phospholipid mixture that reduces alveolar surface tension) from approximately week 34–36 onward. The fetus moves into a head-down (cephalic) presentation in most cases. By week 40, the average fetus weighs approximately 7–8 pounds.

Placental Structure and Fetal Circulation

The placenta is a disc-shaped organ that develops from trophoblastic tissue and the maternal decidua. It allows exchange of oxygen, nutrients, and waste products between mother and fetus without direct mixing of their blood. The umbilical cord contains two umbilical arteries (carrying deoxygenated blood from fetus to placenta) and one umbilical vein (carrying oxygenated blood from placenta to fetus). The amniotic sac encloses the fetus in amniotic fluid, which cushions the fetus, maintains temperature, and allows movement for musculoskeletal development.

Fetal circulation includes three shunts that bypass non-functional fetal lungs and liver: the ductus venosus, foramen ovale, and ductus arteriosus. These close at or shortly after birth when the lungs expand and pulmonary circulation is established.

Maternal Physiological Adaptations

Pregnancy causes profound changes across all body systems:

• Cardiovascular: Blood volume increases by approximately 40–50%. Cardiac output rises. Blood pressure may drop slightly in the second trimester. Heart rate increases. The growing uterus can compress the inferior vena cava in the supine position, causing supine hypotensive syndrome — nurses should position pregnant patients in a left lateral tilt after mid-pregnancy.
• Respiratory: The diaphragm is displaced upward by the growing uterus. Tidal volume increases and respiratory rate may rise slightly; many women experience mild dyspnea.
• Renal: Renal blood flow and glomerular filtration rate increase, which can cause trace glycosuria even in non-diabetic women. The ureters dilate (hydronephrosis of pregnancy), increasing the risk of urinary tract infection.
• Musculoskeletal: Relaxin (a hormone) loosens pelvic ligaments in preparation for birth. The woman's center of gravity shifts forward, causing lordosis and potential low back pain.
• Hematologic: Red blood cell mass increases, but plasma volume increases more, causing physiologic hemodilution (dilutional anemia). White blood cell count rises normally.

Safety Points

Always confirm a patient's last menstrual period and estimated due date before administering any medication, as many drugs are contraindicated in pregnancy. Report any vaginal bleeding, severe abdominal pain, or sudden decrease in fetal movement immediately. Ectopic pregnancy (implantation outside the uterus, most often in the fallopian tube) is a medical emergency — suspect it if a patient presents with unilateral pelvic pain and a positive pregnancy test.