All day numbers given here are the number of days post-ovulation on the conceiving cycle.

The 32 to 64 cell stage of a developing embryo is the morula. This stage is seen up to about day 6 post ovulation.

This photograph is of an expanded blastocyst, highly magnified. Note the regularity of the circular shape. The blastocyst stage of development is considered to be reached when the blastocele cavity, or yolk sac, is seen to be developed within the embryo. It is at this stage of development that embryo transfer is most commonly practised.

A 14 day conceptus viewed by ultrasound. Note the regularity of the circular shape.

16 and 17 DAYS
By 17 days the regularity of shape is starting to deteriorate - note the "ragged" edges now seen on the 17-day pregnancy compared to the 16-day. This is perfectly normal and not generally an indication of a problem - do not give prostaglandin with the intention of starting over, or Regumate (or any progestin) to "maintain the pregnancy"!

By 23 days the embryo itself can be seen with ultrasound - here it is visible between the red calliper marks on the image. The heartbeat can also start to be detected at this stage. Note here how the regular rounding has completely deteriorated, and the conceptus is appearing oval, and without "smooth" edges.

At 33 days, the developing allantois (the outgrowth of the embryo's hindgut which forms the bladder, carries blood vessels in the umbilical cord, and later combines with the chorion to form the placenta) is visible, and the yolk sac (visible here as the upper portion of the dark section) is regressing. The apparent "division" in the conceptus at this stage is not to be mistaken for twin embryos - of which clearly only one is visible.

A different view of a 35 day pregnancy shows that there can be differences of embryo position and yolk/allantoic sac size.

By day 39 the yolk sac has almost completely regressed and is visible here only as the dark circular area immediately above the embryo (compare this with the 33 day image).

A 50 day fetus
By day 50 the embryo officially becomes a fetus. As can be seen in this photograph of a 50-day fetus, the legs, head, tail and eye are all clearly defined externally. Internally organs and skeletal structures are in place. The overall size is a little deceptive with this image, and it is in fact about one inch in length. Within 10 days or so, the sex of the fetus can be determined using ultrasound, by establishing the direction of migration of the pedicle that will become either the penis or the clitoris. Upward migration towards the anus of course indicates a female.

So, are they or not?

Are these twins? The ultrasound at left was done at 21 days, at right 27 days. What do you think?
Click here to get my opinion, and the reasons for it, and the final answer as to whether it was twins or not!

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