LUCKY 'T' RANCH
Fetal Growth and Developement Stages
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 ultrasound; 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 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.
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.
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
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.