Day 24

The vesicle has grown to 1 inch in diameter. It's a shimmering, flabby, translucent bubble with a dark red dot (the embryo) at one end. A network of threadlike blood vessels emanates from the ¼ inch dot. You can barely make out the beginnings of animal features: a head, 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 in a sea of grainy gray. 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 ½ inch in diameter, resting there; this is the embryo. Within the smudge, a tiny black dot, about the size of a pinpoint, will be flashing on and off like a computer's screen's cursor-this is the pea sized embryo's beating heart.

Day 40

The vesicle is now 2 ½ inches in diameter, roughly spherical in shape, and somewhat collapsed. The ¾ 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, and functional elbows an stifle joints. An ultrasound would reveal the vesicle as a roundish black blob: look for the white smudge of an embryo to be suspended from the blob's ceiling, rather than resting on its 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 of Pregnancy

The embryo is now slightly over an inch long, nesting within the confines of the 3-inch vesicle. You can see tiny ribs under its skin; its domed head looks like that of a Chihuahua, and has developed a distinct skull. Little triangles represent its ears; the hock and fetlock joints have developed. At this stage, your future foal officially will graduate from embryo to fetus. On an ultrasound monitor, you'll find the fetus back on the vesicle's floor, due to a lengthening of the umbilical cord. Because of its size-now about that of a pecan-this will be your last opportunity to view the fetus 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 it 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. Its sex is now viable: you can see that little lumps have formed for the scrotum, if it's a male, or the udder, if its a female. The fetus is now about the size of a chipmunk.

Day 100

Your mare's 7-inch fetus is about the size of a 6-week old kitten. You can see a bit of hair on its lips; its ears are unfurling from its head. They're now nearly 1/2 inch long and are curled forward. The coronary bands look like raised lines encircling the tops of its tiny 1/4-inch hooves.

Day 150

Gaining more than a pound every 10 days, the fetus now is about the size of a rabbit. Hair graces its chin, muzzle, and eyelids. If you look closely, you'll see that eyelashes have emerged.

Day 180

The fetus has quadrupled its 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 its chin, throat and muzzle.

Day 270

Your mare's fetus now looks like a foal: fine hair covers its body, and it now has a swatch of hair on its tail. It's about the size of a German Shepherd.

Day 320

In the last week or so, the fetus's lungs have developed to the point that they can function in the "real world"; its legs have strengthened to the point that they can support is weight; and its hair has coarsened, from the fine, silky texture of fetus hair, to that of a bonafide foal. As far as development goes, the fetus is "done." You'll get the chance to meet your mare's foal in a matter of days or weeks. (Normal equine gestation can range from 320 to 365 days.)