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Determination of Gestational Age in Pregnancy

Q: How is gestational age determined during pregnancy?

A: The gestational age of the pregnancy, how far the pregnancy is along,
can be determined accurately early in pregnancy by ultrasound
measurements
of the gestational sac diameter or the length of the fetal
pole (CRL) in order to determine gestational age. Charts have been developed for this purpose, but some simple rules of thumb can also be effectively used.

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).

Crown Rump Length: Gestational age = 6 weeks plus (CRL x days). This relies on the growth of the normal fetus of 1 mm per day after the 6th week of gestation. For example, a CRL of 16 mm would correspond to a gestational age of 8 weeks and two days (6 weeks plus 16 days = 8 weeks and 2 days).

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