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NBME 23 Answers

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A 32-year-old woman comes to the physician after ...

Yolk sac

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The rhombencephalon would be on the actual fetus so just get rid of (D). The "black hole" that the fetus is floating in is the gestational sac so get rid of (C). Now I am no ultrasound expert but I know that the amniotic cavity eventually expands to fuse with the chorion thereby eliminating the chorionic cavity (B). In terms of where the amniotic cavity is shown in this image, I am not sure, so maybe someone can help but this leaves the yolk sac which typically appears within the gestational sac around 5.5 weeks.

kateinwonderland  At the end of the fourth week, the yolk sac presents the appearance of a small pear-shaped opening (traditionally called the umbilical vesicle), into the digestive tube by a long narrow tube, the vitelline duct. (Wiki) +3  
tallerthanmymom  But why does it look completely detached from the fetus? I eliminated yolk sac first because of this +  
makinallkindzofgainz  If you look reeeeeeeally closely, you'll see some signal between the yolk sac and the baby. Although you can't see the entire connection, they are connected. +1  
thotcandy  Pt is roughly 8 weeks pregnant so and typically by 9th week, Amniotic cavity has expended to fill entire volume of Gestational sac. So the entire black part around the fetus is GS/AC. +1  

 +1  upvote downvote
submitted by nwinkelmann(187),

Here's a GREAT video explaining ultrasound findings of pregnancy. @9:57 is a great picture so far with labels of the yolk sac, gestational sack, fetal pole, and amnion. One thing the person explaining said that I thought would be good to keep in mind is that the yolk sac looks like a cheerio inside the gestational sac. Also, this site has a bunch of pictures as well: https://radiopaedia.org/articles/first-trimester?lang=us.

zpatel  what video? +7  
usmlecharserssss  porn video how they make this embryo and yolk sac +  
samsam3711  Shorter video that explains this pretty well: https://www.youtube.com/watch?v=01mMBDEthV8 +  

The gestational sac is spherical in shape, and usually located in the upper part of the fundus of the uterus. By approximately 9 weeks of gestational age, the amniotic sac has expanded to occupy the majority of the volume of the gestational sac, eventually expanding to reduce the extraembryonic coelom to a thin layer between the amnion membrane and the mesoderm. By then, the gestational sac is usually simply called the "amniotic sac".

Development During embryogenesis, the extraembryonic coelom (or chorionic cavity) that constitutes the gestational sac is a portion of the conceptus consisting of a cavity between Heuser's membrane and the Trophoblast.

During formation of the primitive yolk sac, some of the migrating hypoblast cells differentiate into mesenchymal cells that fill the space between Heuser's membrane and the Trophoblast, forming the extraembryonic mesoderm. As development progresses, small lacunae begin to form within the extraembryonic mesoderm which enlarge to become the extraembryonic coelom.

The extraembryonic coelom divides the extraembryonic mesoderm into two layers: extraembryonic splanchnopleuric mesoderm, which lies adjacent to Heuser's membrane around the outside of the primitive yolk sac, and extraembryonic somatopleuric mesoderm, which lies adjacent to the cytotrophoblast layer of the embryo.

The chorionic cavity is enclosed by the chorionic plate, which is composed of an inner layer of somatopleuric mesoderm and an outer layer of trophoblast cells.


 +0  upvote downvote
submitted by thotcandy(17),


this picture will help. The US we were given is somewhere between 3rd and 4th picture.

From what I understand, the whole thing is the gestational sac, including the chorion and everything inside. From there, between the chorion and the amnion/yolk sac is the chorionic cavity.

then you have the amniotic cavity that engulfs the embryo and Yolk sac around the 3rd week. then AC expands and takes up full volume of Chorionic cavity around 9th week.

the yolk stalk is the only remaining thing by 7th week which eventually becomes the umbilical cord. which connects to the chorion/by then the placenta.

So, pt is 8 weeks pregnant, thus AC has pretty much fully expanded, and the yolk sac is tiny with a small sliver of the yolk stalk visible connecting fetus and sac.