Obstetrics and Gynaecology
Conception, Embryology and Placenta
Fertilisation, implantation and placental physiology for MBBS, including hCG, human placental lactogen, amniotic fluid and umbilical cord, mapped to NMC codes OG3 and OG4.
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Conception, Embryology and Placenta
Fertilisation, implantation and placental physiology for MBBS, including hCG, human placental lactogen, amniotic fluid and umbilical cord, mapped to NMC codes OG3 and OG4.
This chapter follows the pregnancy from fertilisation and early embryology to the fully formed placenta, membranes and cord. It explains implantation, the haemochorial placenta and its hormones, amniotic fluid dynamics and the umbilical cord, forming the physiological base for later chapters on foetal wellbeing.
High-yield: Conception, Embryology and Placenta
- Fertilisation normally occurs in the ampulla of the fallopian tube within 24 hours of ovulation.
- The fertilised ovum reaches the uterine cavity as a morula and implants as a blastocyst about six to seven days after fertilisation.
- Implantation is normally in the upper posterior uterine wall; low implantation predisposes to placenta praevia.
- The placenta is haemochorial, meaning maternal blood is in direct contact with the chorionic villi.
- The mature placenta weighs about 500 grams, roughly one-sixth of foetal weight, and has 15 to 20 cotyledons.
- Human chorionic gonadotropin from the syncytiotrophoblast maintains the corpus luteum and is the basis of pregnancy tests.
- The umbilical cord normally contains two arteries and one vein; a single umbilical artery is associated with foetal anomalies.
- The umbilical vein carries oxygenated blood to the foetus, while the two umbilical arteries return deoxygenated blood to the placenta.
- Amniotic fluid volume peaks at about 800 to 1,000 mL near term; it is mainly foetal urine in the second half of pregnancy.
- Polyhydramnios is linked with foetal swallowing problems and maternal diabetes; oligohydramnios with renal agenesis and ruptured membranes.
- The placenta produces human placental lactogen, which has an anti-insulin effect and contributes to gestational diabetes.
- Twins are dizygotic (two ova) or monozygotic (one ovum splitting); chorionicity depends on the timing of the split.
- The decidua is the pregnancy endometrium and is divided into decidua basalis, capsularis and parietalis.
- Wharton's jelly protects the umbilical vessels from compression within the cord.
The placenta and its products, at a glance
- **Type and weight:** Haemochorial; about 500 grams at term with 15 to 20 cotyledons.
- **hCG:** From syncytiotrophoblast; maintains corpus luteum and underlies pregnancy tests.
- **Human placental lactogen:** Anti-insulin action; contributes to gestational diabetes.
- **Umbilical cord:** Two arteries, one vein. Single artery suggests foetal anomaly.
NMC competencies in this chapter
- **OG1.1:** Basis of maternal and foetal physiology in early pregnancy
- **OG3.1:** Physiology of conception and early embryology
- **OG4.1:** Placenta, foetal membranes, amniotic fluid and umbilical cord
Frequently Asked Questions
Where and when does fertilisation occur?
In the ampulla of the fallopian tube, within about 24 hours of ovulation. The blastocyst then implants in the uterus around six to seven days later.
What does a single umbilical artery indicate?
A cord with one artery instead of two is associated with foetal anomalies, especially renal and cardiac, and warrants a careful anomaly scan.
Why does amniotic fluid volume change matter?
Polyhydramnios suggests impaired foetal swallowing or maternal diabetes, while oligohydramnios points to renal problems or ruptured membranes, so both prompt further evaluation.
How is this chapter tested in exams?
hCG and human placental lactogen functions, the haemochorial placenta, cord vessels and amniotic fluid abnormalities are classic single-best-answer facts in obstetrics.
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