The Placenta

The placenta continues to adapt over T2 and T3. It is the primary producer of steroid hormones after 7 weeks' gestational age.

Blood Supply

Flow in the arcuate and radial arteries during normal pregnancy is high with low resistance (resistance falls after 20 weeks).

The decidua produces maternal steroids and synthesizes proteins that are related to the maintenance and protection of the pregnancy from immunologic rejection.

Developmental Ages

Postconception Day


19-21 23-25 25-27

28 44

Tissue/Organ Formation

Blastula Implantation Primitive streak Neural plate First somite

Closure of anterior neuropore Arms bud

Closure of posterior neuropore Legs bud

Sexual differentiation w

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