43.5 Human Pregnancy and Birth

Fetal Development and Pregnancy

Human pregnancy spans approximately 40 weeks from the last menstrual period to birth. During that time, a single fertilized cell develops into a fully formed infant through a tightly coordinated sequence of growth, organ formation, and maturation. Understanding these stages clarifies why prenatal care matters and why certain time windows are especially critical.

Stages of Fetal Development

First trimester (weeks 1–12)

Fertilization occurs when a sperm penetrates the egg, forming a zygote. The zygote undergoes rapid cell division as it travels down the fallopian tube and implants in the uterine wall (around days 6–10).

• During the embryonic stage (weeks 3–8), all major organ systems begin to form: the heart starts beating, the brain and spinal cord take shape, and limb buds appear. This is the period of highest vulnerability to teratogens, which are agents like alcohol, certain drugs, and infections (e.g., rubella) that can cause birth defects.
• The fetal stage begins at week 9. Organs that formed during the embryonic stage now grow and refine. Fingers, toes, and facial features become distinct.

Second trimester (weeks 13–27)

• Bones harden, muscles strengthen, and the nervous system matures enough for the fetus to make noticeable movements (often called "quickening," typically felt around weeks 18–20).
• Fetal hearing develops, and the fetus can respond to external sounds like voices and music.
• Vernix caseosa (a waxy, protective coating) and lanugo (fine downy hair) form on the skin to protect it from the amniotic fluid.

Third trimester (weeks 28–40)

• The fetus gains weight rapidly, accumulating fat stores for insulation and energy after birth.
• Organ maturation accelerates:
  • The lungs produce surfactant, a substance that keeps the alveoli from collapsing when the newborn takes its first breaths.
  • The digestive system develops intestinal villi and enzymes in preparation for nutrient absorption.
• Most fetuses rotate into a head-down position by the final weeks, which allows the easiest passage through the pelvis during delivery.

Fetal Support Structures

Three structures sustain the fetus throughout pregnancy:

• Placenta: An organ that develops from both maternal and embryonic tissue. It exchanges oxygen, nutrients, and wastes between the mother's blood and the fetal blood without the two bloodstreams mixing directly.
• Umbilical cord: The physical connection between the fetus and the placenta, containing two arteries and one vein that carry blood back and forth.
• Amniotic fluid: Liquid filling the amniotic sac that cushions the fetus, maintains a stable temperature, and allows room for movement.

Process of Labor and Childbirth

Gestation (the period from conception to birth) typically lasts about 40 weeks. Labor is triggered in part by rising levels of oxytocin, a hormone that stimulates uterine contractions. Labor proceeds in three stages:

1. First stage: Cervical dilation

   • Latent phase: The cervix thins (effacement) and dilates to about 3–4 cm. Contractions are mild and may be irregular.
   • Active phase: Dilation progresses from about 4 cm to 7 cm. Contractions become stronger, longer, and more frequent.
   • Transition phase: The cervix dilates from 7 cm to the full 10 cm. Contractions are intense and closely spaced. This is typically the shortest but most demanding phase.

2. Second stage: Delivery of the baby

   • The mother pushes to help the fetus descend through the birth canal.
   • Crowning occurs when the widest part of the baby's head becomes visible at the vaginal opening.
   • The head delivers first, followed by the shoulders and the rest of the body.

3. Third stage: Delivery of the placenta

   • Continued uterine contractions separate the placenta from the uterine wall. It is expelled as the "afterbirth," usually within 5–30 minutes of delivery.

Common complications:

• Prolonged labor: Exceeding ~20 hours in a first pregnancy or ~14 hours in subsequent pregnancies.
• Fetal distress: Abnormal fetal heart rate patterns that may require emergency intervention such as cesarean section (surgical delivery).
• Meconium aspiration: The fetus passes its first stool before delivery, and inhaling it can block the airways.
• Shoulder dystocia: The baby's shoulders become lodged behind the mother's pubic bone, delaying delivery and risking oxygen deprivation.
• Postpartum hemorrhage: Excessive blood loss (over 500 mL) after vaginal delivery, which can be life-threatening without prompt treatment.

Effectiveness of Contraceptive Methods

Contraceptive methods vary widely in mechanism and effectiveness. The percentages below reflect perfect use (following instructions exactly) unless otherwise noted. Typical-use rates are often lower.

Hormonal methods

• Combined oral contraceptives contain estrogen and progestin. They work primarily by preventing ovulation. 99% effective with perfect use.
• Progestin-only pills thicken cervical mucus (blocking sperm) and thin the uterine lining. 99% effective with perfect use.
• Intrauterine devices (IUDs) are inserted into the uterus and are over 99% effective.
  • Hormonal IUDs release progestin and last 3–5 years.
  • Copper IUDs release copper ions that are toxic to sperm and last up to 10 years.
• Implants are matchstick-sized rods inserted under the skin of the upper arm. They release progestin and are over 99% effective for up to 3 years.
• Injectable contraceptives (e.g., Depo-Provera) deliver a high dose of progestin every 3 months. About 94% effective with typical use.
• Vaginal rings release estrogen and progestin through the vaginal walls. Replaced monthly; 99% effective with perfect use.
• Patches deliver estrogen and progestin through the skin. Replaced weekly; 99% effective with perfect use.

Barrier methods

• Male condoms (latex, polyurethane, or natural membrane) physically block sperm from reaching the egg. 98% effective with perfect use. They also reduce transmission of sexually transmitted infections.
• Female condoms are inserted into the vagina before intercourse. 95% effective with perfect use.
• Diaphragms and cervical caps cover the cervix to block sperm entry. Diaphragms are about 94% effective; cervical caps about 86% effective with perfect use.

Natural methods

• Fertility awareness methods involve tracking the menstrual cycle (via temperature, cervical mucus, or calendar) to identify fertile days and avoid intercourse during those windows. 76–88% effective with perfect use, depending on the specific method.
• Lactational amenorrhea relies on frequent breastfeeding to suppress ovulation. About 98% effective during the first 6 months postpartum, but only if the mother is exclusively breastfeeding and has not yet resumed menstruation.

Sterilization

• Tubal ligation surgically seals the fallopian tubes, preventing egg and sperm from meeting. Over 99% effective; considered permanent.
• Vasectomy blocks or cuts the vas deferens so sperm no longer enter the semen. Over 99% effective; considered permanent.

Causes and Treatments of Infertility

Infertility is generally defined as the inability to conceive after 12 months of regular, unprotected intercourse. Causes can involve female factors, male factors, or both. In some cases, no clear cause is identified.

Female factors

• Ovulatory disorders such as polycystic ovary syndrome (PCOS) disrupt hormone levels and prevent regular egg release. Treatments include clomiphene citrate (a drug that stimulates ovulation) and metformin (which addresses the insulin resistance commonly associated with PCOS).
• Tubal blockage or damage prevents the egg and sperm from meeting. Surgery can remove scar tissue or repair damaged tubes.
• Endometriosis occurs when tissue similar to the uterine lining grows outside the uterus, forming lesions that cause inflammation and scarring. Laparoscopic surgery can remove the abnormal tissue, and hormonal therapy can suppress estrogen to slow its growth.
• Uterine factors such as fibroids (benign muscle tumors) or polyps can interfere with embryo implantation. Surgical removal can restore a more favorable uterine environment.

Male factors

• Low sperm count or poor motility reduces the chances of fertilization. Intracytoplasmic sperm injection (ICSI), where a single sperm is injected directly into an egg, can overcome severe deficiencies. Donor sperm is another option.
• Varicocele (enlarged veins in the scrotum) raises testicular temperature and impairs sperm production. Surgical repair can improve sperm quality.

Assisted reproductive technologies for unexplained infertility

When no clear cause is found, clinicians often proceed through increasingly intensive interventions:

• Intrauterine insemination (IUI): Concentrated, washed sperm is injected directly into the uterus, bypassing the cervix and shortening the distance sperm must travel.
• In vitro fertilization (IVF) involves several steps:
  1. Controlled ovarian hyperstimulation: Hormones stimulate the ovaries to develop multiple follicles (and therefore multiple eggs).
  2. Egg retrieval and fertilization: Eggs are collected and combined with sperm in the laboratory.
  3. Embryo transfer: One or more resulting embryos are placed into the uterus, where implantation can occur.