🧺 Conception, Fetal Development & the Placenta
Two arteries and one vein, three shunts around the lungs, and the eight-week window when everything can go wrong.
⏰ The timeline that matters
Weeks 3 to 8 is when organs form, so it is when damage becomes permanent - and it is often before a woman knows she is pregnant.
That is the entire argument for folic acid BEFORE conception, and for treating every woman of childbearing age as potentially pregnant when prescribing.
📅 Milestones worth knowing
- Heart begins beating — around week 3–4
- Detectable on ultrasound — about 6 weeks
- Quickening (mother feels movement) — 16–20 weeks
- Viability — around 24 weeks
- Surfactant adequate — 34–36 weeks
🩸 The cord and the placenta
⭐ The cord rule that is backwards
AVA — 2 Arteries, 1 Vein
The umbilical VEIN carries OXYGENATED blood TO the baby. The two arteries carry deoxygenated blood away to the placenta.
It is the opposite of adult circulation, which is exactly why it is asked.
A two-vessel cord - one artery missing - is abnormal and prompts investigation for kidney and other anomalies. Count the vessels at birth.
| Structure | Job | Note |
|---|---|---|
| Placenta | Exchange + endocrine organ | Makes hCG, estrogen, progesterone, hPL |
| Wharton’s jelly | Cushions the cord vessels | Prevents compression |
| Amniotic fluid | Cushions, allows movement, stabilizes temperature | 800–1200 mL at term |
| Amnion / chorion | The membranes | Rupture releases the fluid |
The placenta is a filter that leaks. It passes oxygen, nutrients and maternal antibodies — and also alcohol, nicotine, most drugs and many viruses. Every teratogen question rests on that.
🧡 Fetal circulation — three shortcuts
The lungs are full of fluid and do no work in utero, so blood is routed around them by three shunts.
| Shunt | Bypasses | Route |
|---|---|---|
| Ductus venosus | The liver | Umbilical vein → inferior vena cava |
| Foramen ovale | The lungs | Right atrium → left atrium |
| Ductus arteriosus | The lungs | Pulmonary artery → aorta |
👶 What happens at the first breath
The lungs inflate, pulmonary resistance falls, and pressure in the left side of the heart rises. That pressure change closes the foramen ovale. Rising oxygen levels close the ductus arteriosus.
A patent ductus arteriosus is one of these shunts failing to close — common in preterm infants, and treated with indomethacin or surgery.
🚨 Amniotic fluid abnormalities
- Oligohydramnios (too little) — suggests kidney problems or a leak. The fetus makes the fluid by urinating
- Polyhydramnios (too much) — suggests the fetus cannot swallow: esophageal atresia, neural tube defects; also maternal diabetes
Too little = kidneys. Too much = swallowing.
🎯 NCLEX traps
- Two arteries, one vein — and the vein carries the oxygen
- Weeks 3–8 is the teratogen window
- Folic acid before conception
- Three shunts exist because the lungs do not work yet
- Oligo = kidneys; poly = swallowing