gestational weeks different from ultrasound
The calculator has only one date field: the LMP. When an ultrasound shows a gestational age different from the LMP-based one, there is no second field to "correct" the result, the ultrasound reading needs to be converted into a new reference date and entered in place of the original LMP. This page shows exactly that conversion.
The ultrasound age (79 days) exceeds the LMP-based age (70 days) by exactly 9 days, and that number carries over, unrounded, into both the new LMP and the new EDD.
Here the ultrasound reading (52 days) is smaller than the LMP-based one (56 days), so the new reference date moves forward instead of back, and the EDD follows the same 4-day shift.
Just 2 days of difference are enough to cross the 98-day cutoff that separates the 1st from the 2nd trimester in this calculator, even though the displayed gestational week barely changes.
The gestational count starts at the LMP by universal medical convention, because the exact date of ovulation and fertilization is rarely known precisely. This means that in the first two gestational weeks, pregnancy has not yet occurred biologically, ovulation and fertilization typically happen in weeks 2–3.
Because this calculator resolves everything from a single date, the LMP. To reflect an ultrasound, you work out the adjusted LMP yourself, exam date minus the gestational age it reported, and enter that in place of the original LMP. The formula that runs afterward is identical, only the input date changes.
Yes, because the week and the EDD both come from the same day count applied to the reference date. A 9-day shift in the adjusted LMP produces exactly 9 days of difference in the EDD and changes the total complete gestational days by the same amount, with no extra correction factor.
Yes. The adjusted LMP is calculated by subtracting the reported age from the exam date, so an error of a few days in the exam date carries fully into the new LMP and the resulting EDD. The ultrasound reading alone fixes nothing, it needs the correct exam date to become a reliable adjusted LMP.
Medical notice: this calculation is educational, based on population-level averages, and does not replace prenatal care, ultrasound, diagnosis or guidance from a doctor or obstetrician.
The date you enter stays in your browser only. We do not save or send this data.