Hormones of Pregnancy
Specialized trophoblastic cells invade the endometrium forming endometrial cups around day 35. These cups produce equine chorionic gonadotrophin (ECG) and pregnant mare serum gonadotropin (PMSG). These hormones rise rapidly to 60 days and decline slowly at around 150 days. Under this influence the ovaries continue to show cyclic activity during the first few months of pregnancy. Several corpus luteums are sequentially replaced over the 5-month period. They are a rich source of progesterone and estrogen which helps maintain pregnancy until term and prepare the mammary glands for milk secretion. From the 3rd month throughout the remainder of the pregnancy, the placenta takes over providing high levels of progesterone and estrogen. Relaxin, 1 st produced by the corpus luteum, and later by the placenta increases flexibility of the pubic symphysis and ligaments of the SI joints.
Fetal Development
Day 9 – pinpoint Day 24 – determine head vs. tail, heart beat heard Day 30 – between 1-1.5 cm long Day 40 – 4 legs, eyelids, ears and some joints visible Day 50 – ribs, distinct skull, hock & fetlock joints Day 60 – resembles a hairless horse, hooves, size of football Day 80 – determine sex Day 100 – hair around muzzle
Day 150 – facial hair Day 180 – mane & tail Day 240 – whiskers Day 270 – hair over full body Day 290 – lungs develop Day 320 – full term
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