OG4.1 | Fetal and Placental Development — Summary & Reflection

KEY TAKEAWAYS

Key take-aways from Fetal and Placental Development:

1. Embryonic period (weeks 1–8): All major organ systems are established during organogenesis. Weeks 3–8 are the critical teratogen window; weeks 0–2 are the all-or-nothing period. The fetal period (weeks 9 to term) is dominated by growth and maturation.

1. Fetal milestones: CRL of 5.5–6 cm at 12 weeks; approximately 300 g at 20 weeks; 1000 g at 28 weeks; 3300–3400 g at 40 weeks. The distal femoral ossification centre appears at ~36 weeks.

1. Placental anatomy: Built from chorionic villi (syncytiotrophoblast outer + cytotrophoblast inner layers) bathed in maternal blood in the intervillous space, fed by spiral arteries. Umbilical cord = 2 arteries (deoxygenated, fetus → placenta) + 1 vein (oxygenated, placenta → fetus).

1. Placental physiology — gas exchange: HbF's left-shifted ODC (poor 2,3-DPG binding) + double Bohr effect ensure adequate O₂ delivery at low fetoplacental PO₂. IgG crosses via FcRn receptor-mediated endocytosis; IgM does not.

1. Placental hormones: hCG peaks at 10–12 weeks (rescues corpus luteum, basis of pregnancy tests); hPL is insulin antagonist (drives gestational diabetes physiology); progesterone switches from corpus luteum to placenta at 8–10 weeks (luteo-placental shift); oestrogens (oestriol) require the fetal–placental axis.

1. Factors affecting fetal growth: Genetic > maternal nutrition > placental sufficiency (surface area + spiral artery remodelling + transporter expression) > fetal hormones (IGF-1, IGF-2, insulin) > oxygen > teratogens > maternal disease. Shallow spiral artery remodelling is the basis of pre-eclampsia and IUGR.

1. Clinical applications: Dating by CRL; FGR management requires understanding Doppler physiology; drug choice in pregnancy depends on molecular weight/lipid solubility/protein binding; Rh prophylaxis rationale depends on IgG placental transfer mechanism; PAS (accreta) arises from impaired decidualisation.

REFLECT

Take a few minutes to reflect on what you have learned in this module using Kolb's cycle:

Concrete experience: Imagine you are the obstetrician caring for the 28-year-old woman in the opening scenario — a primigravida at 32 weeks with a small-for-gestational-age fetus, absent end-diastolic flow, and a low-lying placenta. You are preparing to counsel her and decide on further management.

Reflective observation: Which concepts from this module directly inform what you would tell her? Think specifically about: (a) what the Doppler finding of absent end-diastolic flow means in terms of placental resistance, (b) why the placenta might be small and under-perfused, and (c) how this affects fetal oxygen delivery.

Abstract conceptualisation: Now map the clinical scenario to the basic science: draw (mentally or on paper) the connection between impaired spiral artery remodelling → high resistance in the uterine vessels → reduced uteroplacental blood flow → reduced oxygen and nutrient delivery to chorionic villi → impaired fetal growth → redistribution of fetal cardiac output (brain-sparing) → eventually absent/reversed diastolic flow.

Active experimentation: What one piece of information about this specific fetus — one measurable variable from the basic science in this module — would tell you the most about whether fetal oxygen delivery is currently adequate? (Consider: umbilical artery Doppler, middle cerebral artery Doppler to assess brain-sparing, ductus venosus waveform, biophysical profile, fetal scalp pH, or maternal serum hPL as an indirect marker of placental mass.)

Carry this integrative thinking into your antenatal clinic sessions and ultrasound attachment: the basic science you have studied today is the model through which experienced clinicians interpret every growth scan.