Determination of Gestational Age
Measurement of the gestational sac diameter or the length of the fetal pole (CRL) can be used to determine gestational age. Charts have been developed for this purpose, but some simple rules of thumb can also be effectively used.
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Gestational Sac: Gestational age = 4 weeks plus (mean sac diameter in mm x days). This relies on the growth of the normal gestational sac of 1 mm per day after the 4th week of gestation. For example, a gestational sac measuring 11 mm would be approximately 5 weeks and 4 days gestational age. (4 weeks plus 11 days = 5 weeks and 4 days).