Most important events in prenatal development

1. M.A.A.Al-Maqrashi
Prenatal Development Timeline
The First Weak
Day 1 Fertilization resulting in zygote formation
Day 2
Two-cell stage [30 hours or so after fertilization]
Four-cell stage [40 hours or so after fertilization]
Day 3
16-cell stage [morula]
Cell differentiate to Outer & Inner cell mass
Day 4 Blastocyst formation
Day 4-5 Zygote hatching from zona pellucida
Day 6
Decidual Reaction
Trophoblast cells over the embryoblast pole begin to penetrate the uterine epithelial cells
Day 7 Implantation continues
The Second Weak
Day 8
Implantation continues
Trophoblast cells differentiate into two layers: syncytiotrophoblasts & cytotrophoblasts
Production of hCG  + pregnancy test
Bilaminar germ disk formation
amnion and amniotic cavity formation begins
Day 9
Implantation continues
The original surface defect at site of implantation is closed by a fibrin coagulum
Formation of exocoelomic membrane [Houser’s Membrane] & exocoelomic cavity [Primitive yolk
sac]
Syncytiotrophoblast shows a large number of lacunae.
Day 11-12
Implantation completed: blastocyst is shown completely implanted within the endometrium, and
epithelium has regenerated.
Lacunar spaces in syncytiotrophoblast form an inter-communicating network
Maternal capillaries are congested and dilated: sinusoids
trophoblastic lacunae at the embryonic pole are in open connection with maternal sinusoids;
early uteroplacental circulation
Day 12
Extraembryonic mesoderm [EEM] is formed
Small extraembryonic cavities appear within EEM
EEM is split into two by the developing extra-embryonic/chorionic cavity except at the connecting
stalk
Day 13
chorionic cavity enlarges rapidly
Large portions of the exocoelomic cavity are pinched off & secondary yolk sac or definitive yolk
sac is formed
Trophoblastic lacunae are present at the embryonic & abembryonic pole

2. M.A.A.Al-Maqrashi
Formation of primary villi: Cellular columns of cytotrophoblast with the syncytial covering.
The Third Weak
Day 16 Formation of Primary chorionic villi
Primitive streak formation
Gastrulation: formation of trilaminar germ disk
Formation of Notochord
Day 18 Neurulation: Formation of the neural tube by fusion of neural folds
Early Vasculogenesis & Angiogenesis [transitory]
Paraxial mesoderm gets organised into segments [somatomeres]
Day 20 Formation of occipital somite
The Fourth Weak
Main external features are somites and pharyngeal arches
28 Somites
Day 25 Cranial neuropore closes
Day 28 Caudal neuropore closes
The Fifth Weak
42-44 pairs of somites
Forelimbs and hind limbs appear as paddle shaped buds
Primitive umbilical ring
The Sixth Weak
Primitive umbilical cord contains Loops of developing intestine
The Eighth Weak
Age of embryo is expressed as CRL
Increase in head size
Primitive stages of formation of face, eyes, ears
Limb buds show grooves or rays: fingers and toes can be seen
Embryo appears more like “human”
The Twelfth Weak
Loops of developing intestine return to embryonic cavity
Mature umbilical cord