The Crown Rump Length (CRL) measurement in a 6 week gestation.
A mass of fetal cells, separate from the yolk sac, first becomes apparent on transvaginal ultrasound just after the 6th week of gestation. The fetal pole grows at a rate of about 1 mm a day, starting at the 6th week of gestational age.
Yolk sacs larger than 6 mm are usually indicative of an abnormal pregnancy.
Failure to identify (with transvaginal ultrasound) a yolk sac when the gestational sac has grown to 12 mm is also usually indicative of a failed pregnancy.
The rhombencephalon of the developing brain is visible as a prominent fluid space posteriorly.
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