Foal Embryo Development Stages 
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Day 9 - The Embryonic vesicle, which houses the embryo, is visiable to the naked eye. The vesicle is less than a 1/4 inch in diameter, looks like a firm, simmering, translucent bubble.  On the ultrasound screen, you will see it as a black circle in a sea of grainy gray (you mare's uterus). At this point the embryo is no larger than a pin point.
Day 24 - The vesicle has grown to one inch in diameter. It is a simmering, flabby, translucent bubble with a dark red dot (the embryo) at one end. A network of threadlike blood vessels emanates from the 1/4 inch dot.  You can bearly make out the beginnings of animal features: a head, tiny bumps that will become eyes, a fleshy tail nub and four little nubs that will eventually become legs. On the ultrasound monitor you will see the vesicle as an irregular, guitar-pick shaped black blob in a sea of grainy gray. The embryonic heart can be seen on ultrasound screen at this point. To find it, focus on the "floor" surface of the blob. You will see a white smudge, about 1/2 inch in diameter, resting there will be the embryo. Within the smudge, a tiny black dot, about the size of a pinpoint, will be flashing on and off. This is the pea sized embryo's beating heart.
Day 40 - The vesicle is now 2 1/2 inches in diameter, roughly spherical in shape, and somewhat collapsed. The 3/4 inch embryo within is now recognizable as a four-legged critter:  it has a blobby dome for a head, eyelids, rudimentary ears, ridges where the nostrils will be, functional elbows and stifle joints. An unltrasound would reveal the vesicle as a roundish black blob: look the white smudge to be suspended from the blob's ceiling, rather than resting on it's floor. This shift of position is step one of what researchers call, " the rise and fall of the embryo".  It results from filmy membranes at the top of the vesicle coming together to form the umbilical cord, as they do so, they shorten, pulling the olive sized embryo up to the ceiling like a chandelier
Day 50 to 55 - The embryo is now slightly over an inch long, nesting within the confines of an 3 inch vesicle. You can see tiny ribs under it's skin, it's domed head looks like that of a chihuahua and it has developed a distinct skull. Little triangles represent it's ears, the hock and fetlock joint have now developed. At this stage your future foal has graduated from embryo to fetus. On a ultrasound monitor, you will find the fetus back on the vesicle floor, due to the lenghtening of the umbilical cord. Because of it's size now, that of a pecan, this will be your last chance to view via ultrasound. In a matter of weeks it'll be too large for the screen.
Day 60 - The vesicle is now flabby and shapeless, conforming to the uterine walls; the fetus is about 2 1/2 inches long. You can see that is clearly resembles a horse, thanks to the development of tiny hooves, complete with soles and frogs. Its head is still tucked, but less so than before. The fetus is hairless and about the size of a hamster.
Day 80 - The fetal head and neck will be untucked and are being held level with the spine in the "normal" horse position. It's sex is now visable: you can see that little lumps have formed for the scrotum, if it's a male, or the udder if it's a female. The fetus is now about the size of a chipmunk.
Day 100 - Your mare's seven inch fetus is about the size of a six week old kitten. You can see a bit of hair on it's lips; it's ears are unfurling from it's head. They are now nearly an inch long and curled forward. The coronary bands look like raised lines encircling the tops of it's tiny 1/4 inch hooves.
Day 150 - Gaining more than a pound every 10 days, the fetus is now about the size of a rabbit. Hair graces it's chin, muzzle and eyelids. If you look closely, you will see that eyelashes have emerged.
Day 180 - The fetus has quadrupled it's own weight in just 30 days. Mane and tail hairs have appeared; It's about the size of a beagle.
Day 240 - Now about the size of a small lamb, the fetus has whisker-like hairs on it's chin, throat and muzzle.
Day 270 - Your mare's fetus now looks like a foal: fine hairs cover it's body and it now has a swatch of hair on it's tail. It's about the size of a German Shepard.
Day 320 - In the last week or so, the fetus has fully developed. It's legs have strengthened to support it's own weight and it's hair has coarsened, from the fine, silky texture of fetus hair to that of a bonafide foal. As far as development is concerned, the fetus is "done".  You will get your chance to met your foal in a matter of days or weeks. Normal equine gestation is 320 to 365 days.