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For more details go to the following link: The Fetal Medicine Foundation.
Initially twins may be identified as 2 separate gestational sacs (ie diamniotic, dichorionic) They may be 2 fetal poles within the same gestational sac (monochorionic).
After 12 weeks, the accuracy of CRL in predicting gestational age diminishes and is replaced by measurement of the fetal biparietal diameter.
In at least some respects, the term "crown rump length" is misleading: The normal small mid-gut hernia into the cord is still visible.(pink shading).
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.
Thus, a simple way to "date" an early pregnancy is to add the length of the fetus (in mm) to 6 weeks.
Sometimes there is difficulty distinguishing between the maternal pulse and fetal heart beat.
Early ultrasonographers used this term (CRL) because early fetuses also adopted the sitting in the chair posture in early pregnancy.In the early scans at 5-6 weeks just visualising a heart beating is the important thing.Failure to identify fetal cardiac activity in a fetus whose overall length is greater than 4 mm is an ominous sign.In these cases, one of the twins fails to grow and thrive.Instead, its development arrests and it is reabsorbed, with no evidence at delivery of the twin pregnancy.