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About this tool
Work out which estimated due date governs when the LMP date and the ultrasound date disagree, using the ACOG Committee Opinion 700 redating thresholds.
This page decides which of two disagreeing estimated due dates governs a pregnancy: the one from the last menstrual period, or the one from an ultrasound. It calculates the LMP due date by Naegele's rule (LMP plus 280 days, shifted by cycle length minus 28), calculates the scan due date from the gestational age the scan measured, then applies Table 1 of ACOG/AIUM/SMFM Committee Opinion 700, which says the ultrasound replaces the LMP date only when the gap exceeds 5 days up to 8w 6d, 7 days from 9w 0d to 15w 6d, 10 days from 16w 0d to 21w 6d, 14 days from 22w 0d to 27w 6d, or 21 days from 28w 0d onwards. It is for anyone holding a booking-appointment date and a scan report that do not match and wanting to know, by the published rule, which one the notes should carry. It reports what the rule produces and stops there: no risk reading, no clinical direction.
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Enter the values you already know.
Fine-tune the options to match your scenario.
Read the result and use it in your planning or reporting.
Obstetric practice never averages the two dates. Committee Opinion 700 Table 1 either redates the pregnancy or leaves it alone, and this page shows which row applied and by how many days the gap missed or cleared it.
Table 1's gestational-age column is footnoted 'Based on LMP', so the band is picked from the LMP-based gestational age on the day of the scan, not from the scan's own figure. The page shows both so you can see when they sit in different rows.
Enter the weeks and days from the report, or enter the crown-rump length in millimetres and let the Robinson-Fleming 1975 equation convert it, valid over 10 to 84 mm.
Trimester boundaries at 13w 6d and 28w 0d, the ACOG/SMFM periviable window of 20w 0d to 25w 6d, and the Committee Opinion 579 term ranges, all stamped with calendar dates from the governing due date.
The scan. Below 9w 0d, Committee Opinion 700 Table 1 redates the pregnancy when the gap is more than 5 days, and 6 days clears that. At exactly 5 days the LMP date is kept, because the rule reads 'more than 5 days', not 'five days or more'.
Because the tolerance widens with gestational age. Crown-rump length measured before 9 weeks is accurate to about 5 to 7 days, so a 10-day gap is beyond what measurement error explains. By 16w 0d to 21w 6d the head, abdomen and femur measurements only pin the date to about 10 days, so a 10-day gap is inside the expected spread and the LMP date stands.
Naegele's rule: first day of the last menstrual period plus 280 days, which is 40 weeks. That assumes a 28-day cycle with ovulation on day 14, so a 35-day cycle pushes the due date 7 days later and a 24-day cycle pulls it 4 days earlier. This page applies that shift whenever the cycle length entered is not 28.
ACOG Committee Opinion 579 splits it four ways: early term is 37w 0d to 38w 6d, full term is 39w 0d to 40w 6d, late term is 41w 0d to 41w 6d, and post term is 42w 0d onwards. Anything before 37w 0d is preterm. All four ranges are dated on this page from whichever due date governs.
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