Early Embryonic Death
For all horse breeders, it is important to realize that not every mare will give birth to a live foal. Failure of the pregnancy can occur at any stage starting from the fertilization of the oocyte or egg. The oocyte develops in the fluid- filled
For all horse breeders, it is important to realize that not every mare will give birth to a live foal. Failure of the pregnancy can occur at any stage starting from the fertilization of the oocyte or egg. The oocyte develops in the fluid- filled follicle (Figure One). When it is mature, the oocyte is shed from the follicle into the fallopian tube. This is called ovulation. The fallopian tubes, or oviducts, stretch from the uterus to the ovaries. If the mare has been bred by a stallion or inseminated in the last two or three days, the fallopian tubes should contain sperm (Figure Two). The sperm will burrow into the oocyte and fertilize it to produce the early embryo.
In the mare, fertilization occurs in the fallopian tube and can take place through either natural or artificial insemination. It is a unique event that brings the genetic material of the parents together. It also is a very complex process in terms of both timing and management. Sperm normally live only two or three days in the fallopian tube, so the mare should not be bred too early in estrus or the sperm could be dead before the oocyte arrives in the fallopian tube. You might wonder how the sperm and oocyte ever manage to meet, but statistics show that pregnancy rates average out at 70% by the end of the season.
The fertilized embryo remains in the oviduct for five to six days. After that time, it leaves the oviduct and enters the uterus. The equine oviduct appears to have the unique ability to retain unfertilized ova. This discovery was made as long ago as 1966 and has been confirmed by many subsequent studies. The mechanism remains unclear to this day.
The next stage in the reproductive process is gestation, which begins when the oocyte is fertilized and ends at birth. During that time, the fetus develops and grows to a state where the newborn foal can survive in the outside world. There are hormonal changes during pregnancy; in particular the hormone progesterone continues to be produced.
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