Some of you may be wondering what the foals (fetuses, really) look like today. If they are developing normally, at this stage - 170 days - they should be almost the size of a Beagle. Mane and tail hairs are starting to appear and they should look something like this:How did they get to this stage?
Day 9. With the naked eye you can only see the 'embryonic vesicle ' which houses the embryo. The vesicle looks like a shimmering , firm translucent bubble less than 1/4 " in diameter. On the ultrasound screen, you will see it as a black circle in a sea of grainy grey (the mares uterus).At this point the embryo is only the size of a pinpoint. 
Day 24. The vesicle has grown to 1" in diameter. It is a shimmering flabby, translucent bubble with a dark red dot, the embryo, at one end. A network of thread like blood vessels emanates from the 1/4" dot. You can barely make out the beginnings of the animal features. A head with tiny bumps that will become eyes, a fleshy tail nub, and four little buds that will eventually become legs. On the ultrasound monitor, you will see the vesicle as an irregular, guitar pick shaped black blob within the grainy grey. Generally around day 24, an embryonic heart is large enough to be seen on the ultrasound screen. To find it, focus on the 'floor' surface of the blob. You will see a white smudge about 1/2" in diameter, resting there is the embryo. Within the smudge, a tiny black spot, 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" in diameter, roughly spherical in shape and somewhat collapsed. The 3/4 inch embryo within is now recognisable as a four legged animal. It has a blobby head with eyelids, rudimentary ears, ridges where the nostrils will be, and functional elbows and stifle joints. An ultrasound would reveal the vesicle as a round black blob. Look for the white smudge of the embryo to be suspended from the vesicle's ceiling, rather than resting on the floor. This shift of position is step one in 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 now is slightly over an inch long nestling within the confines of the 3inch vesicle. You can see tiny ribs under the skin, the domed head is similar to a 
Day 60. HAMSTER The vesicle is now flabby and shapeless conforming to the uterine walls. The fetus is now approx 2 1/2 inches long. You can see that it clearly resembles a horse thanks to the development of tiny hooves, complete with soles and frogs. It's head is still tucked but less than before. The fetus is hairless and about the size of a hamster.
Day 80. CHIPMUNK The fetal head will now be un-tucked and is held level with the spine in the 'normal' horse position. It's sex is now visible. You can see that little lumps have formed for the scrotum if it's a colt or the udder if it is a filly. The fetus is now about the size of a chipmunk.
Day 100. KITTEN The mares 7inch fetus is now about the size of a six week old kitten. You can see a bit of hair on it's lips and it's ears are uncurling from it's head, they are now nearly 1/2 inch long and curled forwards on it's head. The coronary band looks like little raised lines circling the tops of it's tiny 1/4 inch hooves.
Day 150. RABBIT 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 eye lids. If you look closely, you can see the eyelashes have emerged.