---
module: 085-03
language: en
chapter: 85
title: "Developmental Biology, Germ Layers, Folding, and Placentation"
module_title: "Placenta, fetal membranes, maternal–fetal exchange, twins, and developmental timing"
source_sha256: 1cdbef86470c4a2546c6862ce494eece66183f5fa041e247f97f41a23a565cc8
---
# Placenta, membranes, twins, and timing

## Placental structure
### Fetal chorion plus maternal decidua
### Blood approaches without bulk mixing
### Primary, secondary, tertiary villi
#### Cytotrophoblast columns, then mesoderm core
#### Fetal capillaries make tertiary villi
### Anchoring villi attach to decidua
### Floating terminal villi provide exchange
### Barrier layers under syncytiotrophoblast
#### Hofbauer macrophages in villous core

## Exchange
### Barrier thins as pregnancy advances
#### Shorter diffusion distance
### Gases diffuse
### Glucose by facilitated transport
### Amino acids by active or carrier transport
### Immunoglobulin G by receptor-mediated transcytosis
### Oxygen transfer determinants
#### Partial pressure, area, thickness, flow
#### Fetal haemoglobin, double Bohr effect
### Placenta consumes oxygen itself
### Not a simple filter
#### Waste moves to maternal circulation

## Maternal blood supply
### Spiral arteries feed intervillous space
### Extravillous trophoblast remodels arteries
#### Wide low-resistance channels
### Incomplete remodelling
#### Growth restriction and pre-eclampsia
### Umbilical arteries carry deoxygenated blood

## Endocrine and immune roles
### Chorionic gonadotropin maintains corpus luteum
### Progesterone suppresses contractility
### Oestriol needs fetal adrenal and liver
### Placental lactogen shifts maternal metabolism
#### Insulin resistance spares fuel for fetus
#### Excess: gestational diabetes
### Corticotropin-releasing hormone times birth
### Interface is immunologically active
#### Some histocompatibility molecules absent
#### Uterine natural-killer cells and macrophages
#### Adjusted, not globally suppressed

## Decidua and membranes
### Decidua basalis forms maternal part
### Capsularis fuses with parietalis
### Chorion frondosum persists, laeve regresses
#### Discoid placenta
### Cotyledons from decidual septa
### Inspect placenta after delivery
### Amniotic fluid: urine in, swallowing out
#### Oligohydramnios: pulmonary hypoplasia
#### Polyhydramnios: impaired swallowing, diabetes
### Allantois forms urachus
#### Remnant: median umbilical ligament
### Cord: two arteries, one vein, Wharton jelly

## Fetal circulation
### Ductus venosus bypasses liver
### Foramen ovale shunts right to left
#### Oxygenated blood to heart and brain
### Ductus arteriosus bypasses lungs
#### High pulmonary resistance
### Changes at birth
#### Lung expansion lowers pulmonary resistance
#### Left-atrial pressure closes foramen ovale
#### Oxygen and low prostaglandins constrict ductus

## Twins
### Dizygotic: two oocytes, usually dichorionic
### Monozygotic type depends on split timing
#### Very early: dichorionic diamniotic
#### Blastocyst: monochorionic diamniotic
#### Later: monochorionic monoamniotic
#### Incomplete late split: conjoined
### Chorionicity outweighs zygosity
#### Twin-to-twin transfusion
#### Monoamniotic cord entanglement

## Timing and teratogens
### Crown-rump length accurate early
### Growth restriction versus small healthy fetus
### First two weeks: all-or-none, not absolute
### Weeks three to eight: organogenesis
### Fetal period: growth, ongoing CNS risk
### Outcome depends on dose, timing, genotype
### Communicate absolute risk and uncertainty