With the naked eye, you can see only the "embryonic vesicle" which
houses the embryo. The vesicle looks like a shimmering, firm, translucent
bubble, less than ¼ inch in diameter. On the ultrasound screen, you will see it
as a black circle in a sea of grainy gray (your mare's uterus). At this point,
the embryo is no larger than a pinpoint.
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.
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
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 ultrasonography; in a matter of
weeks, it'll be too large for the screen.
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 developemnt 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.
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 visable: 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 chimpmunk.
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.
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.
The fetus has quadrupled its weight in just 30 days. Mane and tail hairs have
appeared; it's about the size of a Beagle.
Now about the size of a small lamb, the fetus has whisker-like hairs on its
chin, throat and muzzle.
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.
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 mares's foal in a matter of days or weeks. (Normal equine gestation can
range from 320 to 365 days.)