- Stage one, cleavage, occurs in the first five days after fertilization. The embryo is less than the size of a pinhead.
- Stage two, differentiation, starts cellular specialization into internal organs; the skeletal, skin and muscular systems; and circulatory and reproductive organs. This process lasts 40 days.
- Stage three, growth, lasts from day 45 to day 335. The fetus will grow 12 times in size, and during the last two months of gestation, will gain additional mass.
- The complete estrous cycle in mares will average 21 days. Gestation, from fertilization to birth, will run approximately 335 days, with variations of plus or minus 45 days considered normal.
- External factors can affect the duration and outcome of gestation in mares. Cold, dry climates can add five to six days to the gestation period, and an older or malnourished mare can add 10 to 15 days. Colts often gestate eight to 10 days longer than fillies, and larger breeds often gestate longer than smaller breeds.
- Prolonged labor (average is two to three hours), vaginal bleeding, placental tear or abnormal position of the fetus require immediate veterinary attention to protect the mare and foal.
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