Friday, January 4, 2013



Fetus Development of the Horse

By Catherine Sampson

The remarkable journey of the unborn foal has been well documented.  From the moment of conception, development of the full bonafide horse is met with extraordinary discovery. 

At just nine days after conception the tiny fetus measures a quarter of an inch. Its circulatory system has been established and is now on its transforming journey of epic proportions. 

At the two week stage of development the heart is now present and blood begins to circulate.  Another seven days later and the circulating blood is clearly visible to the naked eye.

In twenty-four days the beating heart is detectable by ultrasound examination.  At this point the head, eyes, tail and limbs are forming.  Soon a recognizable form will start to grow with definition.

Just over a month in development at forty days the fetus has grown to two and one-half inches and the reproductive system is visible.  Sex determination is evident.  By this stage the head, eyelids, ears, elbows and stifle joints are recognizable.  The fetus now weighs half an ounce.  Still a far cry from its birth weight, our little foal is now undeniably going through a metamorphosis of remarkable occurrence.

Between fifty and fifty-five days, tiny ribs, hocks, fetlock joints and ears begin to appear.  Almost all of the internal organs are now present.  It is also the last opportunity to examine the fetus via ultra sound.  Its size will soon be too large to examine except through rectal palpation.

At two months into development, the fetus is now two and one half inches in length.  It has also doubled in weight to one ounce.  It is clearly defined as a horse complete with hooves, sole and frogs.

In eighty days the sex of the horse can be easily determined although not possible except in post-mortem.  The fetus is rapidly developing averaging a size of four inches in length.  The head and neck are held level with the spine in a normal plane.

At just one hundred days from conception tiny hairs are forming on the lips.  The ears are unfurling.  Coronary bands resemble raised lines on the one-quarter inch hooves and the fetus has reached a size of seven inches.  It weighs one pound.

At one hundred and fifty days the fetus is gaining weight rapidly now averaging more than one pound every ten days.  Hair appears on its chin, muzzle and eyelids and eyelashes are starting to form.  The fetus is comparable in size to that of a rabbit and weighs out at six pounds.  A mere thirty days later the fetus has quadrupled its weight.  Its mane and tail hairs appear and the fetus now weighs a hefty twenty-five pounds.

With the mare in her last trimester of gestation diet will increase to accommodate her quickly maturing fetus.  A vitamin supplement should be added to her grain ration to assist her depleting reserves.  This is also the time to booster your mare’s tetanus vaccine.  This will provide additional protection to your unborn foal.  You should also ask your veterinarian about parasite control of your mare during this last trimester.  Your veterinarian may recommend a specific deworming program prior to the last forty days of gestation.  Certain deworming products are not appropriate for pregnant mares during this crucial period. 

Two hundred and four days into development the fetus has stretched to two feet in length and weighs forty-five pounds.  Thirty days later a fine hair coat is covering the body and a swatch of hair appears on the tail.  In this short period of time the fetus has gained a further thirty pounds.

As the fetus races to its foaling date the final development stages are accelerating.  One of the last organs to develop is the lungs.  This is an important stage in the development.  Premature foals usually don’t survive, not because they are not strong enough, but because their lungs are not functioning. 

When the big day arrives the emerging foal will weigh in the neighbourhood of one hundred pounds.  The ears may seem limp at first.  In order for this cartilage to strengthen, the newborn foal must use its ears.  The same is true for its shaky, sometimes weak limbs and joints.  The foal’s newfound freedom and exercise are now critical to their continuing development outside the womb. 

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