Pregnancy, Fetal Development, and Birth

The placenta. Once implanted, the embryo develops the placenta, a temporary organ that connects the developing fetus (via the umbilical cord) to the mother's blood supply. The placenta:

• Exchanges materials: oxygen and nutrients pass from mother to fetus, and wastes (like carbon dioxide) pass back — without the two bloodstreams directly mixing.
• Produces hormones (including hCG, progesterone, and estrogen) that maintain pregnancy.
• Provides some protection, though many substances (including harmful ones — teratogens) can cross it, which is why maternal health and exposures matter greatly.

Recall that the fetus doesn't breathe air, so its lungs are largely bypassed (fetal circulation shunts) and the placenta does the "breathing" — connecting back to the circulatory pathways unit.

Development across trimesters. Pregnancy (about 40 weeks) is divided into three trimesters:

• First trimester: the embryonic period and early fetal period — the major organs and body systems form (organogenesis). This is the most vulnerable time for developmental harm from teratogens.
• Second trimester: the fetus grows and systems mature; movement is felt.
• Third trimester: rapid growth and final maturation (especially of the lungs) prepare the fetus for life outside the womb.

Hormones and labor. Near the end of pregnancy, hormonal and physical changes prepare for birth (parturition). Labor is famously driven by a positive-feedback loop (recall from homeostasis): stretching of the cervix triggers release of oxytocin, which strengthens uterine contractions, which stretch the cervix more, releasing more oxytocin — an escalating cycle that continues until the baby is delivered, ending the stimulus. This is one of the body's few positive-feedback systems, used precisely because it needs to build to a clear endpoint.

Stages of birth. Labor proceeds in three stages:

1. Dilation stage: the cervix dilates (opens) fully; contractions become stronger and more frequent (the longest stage).
2. Expulsion stage: the baby is pushed through the birth canal and delivered.
3. Placental stage: the placenta (afterbirth) is delivered shortly after the baby.

After birth, the newborn begins breathing (the fetal circulatory shunts close), and hormonal changes support lactation (oxytocin also drives milk ejection, recall the endocrine unit).

How it works

Pregnancy to birth:

Implantation → placenta forms (exchange O₂/nutrients/wastes; makes hormones)
Trimesters: 1st (organs form — vulnerable) → 2nd (grow/mature) → 3rd (final growth, lung maturation)
Labor: cervix stretch → oxytocin → stronger contractions → more stretch → more oxytocin (positive feedback) → delivery
Birth stages: dilation → expulsion (baby) → placental (afterbirth)

Comparisons

Common confusions

• The placenta exchanges materials without mixing the two bloodstreams.
• First trimester = organ formation (most vulnerable to teratogens); later trimesters = growth/maturation.
• Labor uses positive feedback (oxytocin) — one of the body's few, building to delivery.
• The placenta ("afterbirth") is delivered after the baby (third stage).

Memory aids

• "Placenta = the fetus's lungs, gut, and kidney combined (via mom)."
• Labor stages: "Dilation, Delivery, Detachment (placenta)."
• Oxytocin = "the labor + milk-ejection hormone (positive feedback)."

Quick review

• The placenta (via the umbilical cord) exchanges oxygen, nutrients, and wastes between mother and fetus (without mixing blood) and makes pregnancy hormones; many substances, including teratogens, cross it.
• Pregnancy's three trimesters: organ formation (first, most vulnerable), growth/maturation (second), and final growth + lung maturation (third).
• Labor is driven by a positive-feedback oxytocin–contraction loop; birth stages are dilation → expulsion → placental (afterbirth).
• Prenatal care, teratogen avoidance, oxytocin's role, and newborn transition are key clinical points.