The Reproductive System and Pregnancy Physiology

Why This One Is on the List

Reproductive anatomy earns its place for two reasons. Obstetric calls happen, and the physiology of pregnancy changes how a patient presents so substantially that reading her vital signs against non pregnant expectations will mislead you.

Male Reproductive Anatomy

Testes. Two, held outside the body in the scrotum, which keeps them slightly below core temperature because sperm production requires it. They produce sperm and testosterone.

Epididymis. A coiled tube on the back of each testis where sperm mature and are stored.

Vas deferens. The tube carrying sperm from the epididymis up into the pelvis toward the urethra.

Prostate. A gland surrounding the urethra just below the bladder. It contributes fluid to semen. It enlarges with age in most men, which narrows the urethra and produces difficulty urinating and incomplete emptying.

Seminal vesicles and bulbourethral glands. Additional glands adding fluid to semen.

Penis. Contains erectile tissue and carries the urethra, which serves both urinary and reproductive functions in males.

Two conditions worth knowing. Testicular torsion is a twisting of the testis on its own blood supply, producing sudden severe scrotal pain, often in adolescents, and it is time critical because the testis loses its circulation. Priapism, a prolonged erection unrelated to arousal, can accompany sickle cell disease, certain medications, and spinal cord injury.

Female Reproductive Anatomy

Ovaries. Two, in the lower abdomen on either side. They produce eggs and the hormones estrogen and progesterone. A female is born with all the eggs she will ever have.

Fallopian tubes. Carry the egg from the ovary to the uterus. Fertilization normally happens inside the tube. An embryo implanting there instead of in the uterus is an ectopic pregnancy, and because the tube cannot expand, it can rupture and bleed severely. Lower abdominal pain in a female of childbearing age is treated as a possible ectopic pregnancy until it is ruled out elsewhere.

Uterus. A muscular organ in the pelvis where a fetus develops. The endometrium is its inner lining, built up and shed each cycle. The myometrium is the thick muscular wall that contracts during labor and clamps down afterward to control bleeding. The cervix is the lower narrow portion opening into the vagina.

Vagina. The canal from the cervix to the outside, serving as the birth canal.

Perineum. The tissue between the vaginal opening and the anus, which stretches during delivery and can tear.

The Menstrual Cycle in Brief

The cycle runs roughly 28 days, though the range is wide.

Rising estrogen rebuilds the endometrium in the first half. Around the midpoint, ovulation releases an egg from an ovary. Progesterone then maintains the lining in preparation for implantation.

Without fertilization, both hormones fall, the lining is shed as menstruation, and the cycle restarts. With fertilization, hormone production continues and the lining is maintained.

Knowing the timing has one practical use: the date of the last menstrual period is what estimates a due date, and it is a question worth asking.

Pregnancy Structures

Placenta. A temporary organ forming on the uterine wall, connecting maternal circulation to fetal circulation. It exchanges oxygen, nutrients and waste between two blood supplies that never actually mix, and it produces the hormones that maintain the pregnancy. It is expelled after the baby, as the third stage of labor.

Umbilical cord. Connects fetus to placenta. Two arteries carry blood from the fetus toward the placenta, and one vein carries oxygenated blood back to the fetus. The naming follows direction of flow, not oxygen content, consistent with the rest of the circulatory system.

Amniotic sac and fluid. The membrane surrounding the fetus and the fluid inside it, which cushions the fetus, allows movement and maintains temperature. Roughly 500 to 1,000 mL at term. Rupture of the sac is the water breaking.

Pregnancy is divided into three trimesters of about thirteen weeks each, with full term at 37 to 40 weeks.

Physiologic Changes of Pregnancy

These are the changes that alter how a pregnant patient presents, and they are the reason a pregnant trauma patient is assessed differently.

Blood volume rises 40 to 50 percent by the third trimester. Plasma increases more than red cell mass does, which makes the hematocrit read low without meaning anemia in the usual sense.

The consequence matters enormously: she can lose a substantial volume of blood before her vital signs change. A normal blood pressure in a bleeding pregnant patient is not reassurance. The fetus is also not protected by that reserve, because maternal compensation shunts blood away from the uterus early to protect the mother's core. Fetal distress can precede any maternal sign.

Heart rate rises by roughly 10 to 20 beats per minute at baseline. Blood pressure falls slightly in the second trimester and returns toward normal near term.

Cardiac output rises by 30 to 50 percent.

Tidal volume rises while the respiratory rate changes little. The growing uterus pushes the diaphragm upward, which reduces the reserve she has available. Oxygen consumption is higher and she desaturates faster.

Gastric emptying slows and the lower esophageal sphincter relaxes, so a pregnant patient is at higher risk of vomiting and aspiration.

Uterine blood flow becomes substantial, which is why uterine injury or a placental problem can bleed heavily.

Supine Positioning Late in Pregnancy

By the third trimester the uterus is heavy enough that lying flat presses it against the inferior vena cava, which runs along the spine. Compressing that vessel cuts the blood returning to the heart, which drops cardiac output and blood pressure. The patient becomes lightheaded, pale and nauseated, and it resolves when she moves.

Positioning her on her left side takes the weight off the vessel. When she has to remain flat, displacing the uterus manually to the left accomplishes the same thing.

Labor

Three stages.

First stage runs from the onset of regular contractions until the cervix is fully dilated. It is the longest stage, especially with a first pregnancy.

Second stage runs from full dilation until the baby is delivered. Contractions are strong and close together and the mother feels an urge to push.

Third stage is delivery of the placenta, usually within about thirty minutes of the birth.

Crowning, when the baby's head is visible at the vaginal opening, means delivery is imminent.

The Newborn

Assessment centers on three things: appearance, breathing effort and heart rate. Most newborns need only drying, warming, positioning and stimulation.

The APGAR score is assessed at one minute and again at five minutes, scoring five parameters from 0 to 2 for a maximum of 10: appearance (color), pulse, grimace (reflex irritability), activity (muscle tone) and respiration. The one minute score describes the transition, and the five minute score describes the response to whatever was done.

A newborn heart rate below 100 with inadequate breathing calls for ventilation. Newborns lose heat very quickly because of their surface area relative to their mass, so drying and covering the head is genuine treatment rather than comfort.

Complications Worth Recognizing

Preeclampsia is high blood pressure developing in pregnancy, with swelling, headache and visual changes. Progression to seizures is eclampsia.

Placenta previa is a placenta covering the cervix, producing painless bright red bleeding.

Placental abruption is the placenta separating from the uterine wall early, producing pain and a rigid uterus, with bleeding that may be concealed inside the uterus rather than visible.

Postpartum hemorrhage is excessive bleeding after delivery, most often because the uterus fails to contract down. Uterine massage and letting the newborn nurse both encourage that contraction.

Abnormal presentations, including a prolapsed cord, a breech, or a shoulder that will not deliver, are finished at the hospital. No level of prehospital provider carries what those patients need. Position and protect what you can while you are moving, and transport, with notification en route.

What to Take Away

The placenta exchanges gas and nutrients between two circulations that never mix. Blood volume rises 40 to 50 percent, so vital signs stay normal well into blood loss and a normal pressure proves nothing. Cardiac output and tidal volume rise, reserve falls, and she desaturates quickly. Flat on her back compresses the vena cava, so tilt her left. APGAR is scored at one and five minutes. An abnormal presentation is transported, not managed on scene.