Dating ultrasound measurements
METHODS Urban areas in eight geographically diverse countries that met strict eligibility criteria were selected for the prospective, population…
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We restricted our analysis to those who had fresh embryo transfer since we were concerned with the need to add the time from egg pickup to freezing, together with the time from subsequent thawing to transfer might introduce inaccuracies.
We determined gestational age on the day of the ultrasound (IVF dates) by nominating the day of egg pickup and fertilisation as day 14 of gestation. Note that some charts did not have corresponding gestational ages for all measurements which accounts for the variability in sample size seen in the comparison of charts.
To validate our chart, we obtained CRL measurements from an IVF twin cohort (fresh embryo transfer) at 6 ( 1d) to 9 ( 0d) weeks of gestation where gestational age was calculated (fertilisation age 14 days).In the first trimester, there is a very little biologic variation in fetal size compared with later trimesters.It is therefore a good time in pregnancy to determine gestational age by ultrasound where the crown-rump length (CRL) is measured and compared to published reference charts.In contrast, dates calculated from the first day of the last menstrual period (menstrual age) may have inaccuracies arising from imprecise recollection of dates, variation in the timing of ovulation, or time to conception.
A number of CRL reference charts have been proposed and different versions are in common use (Table 1).
It should be possible to generate very accurate reference charts by using a large in vitro fertilisation (IVF) cohort where CRL measurements could be correlated with gestational age precisely calculated from date of fertilisation.