Pregnancy Due Date Redating Calculator
Enter the gestational age from the current dating method and the gestational age reported by ultrasound. The calculator compares the difference with ACOG redating thresholds and shows whether the entered dates point to routine redating, no routine change, or clinician review.
Redating recommendationUse ultrasound dating because the discrepancy is greater than the ACOG threshold.
- Dating discrepancy
- 6 days; ultrasound dating is ahead of the current dates.
- ACOG redating threshold
- More than 5 days before 9 0/7 weeks
- Estimated EDD adjustment
- Move the EDD 6 days earlier.
- Resulting gestational age
- 8 6/7 weeks
- Dating-quality flag
- No suboptimally dated flag from these entries.
This calculator applies gestational-age discrepancy thresholds; it does not derive gestational age from ultrasound measurements.
The ACOG thresholds use a strict more-than rule, so a discrepancy equal to the cutoff does not trigger routine redating.
How to use this calculator
- Choose how the current gestational age or EDD was established.
- Enter the completed weeks and extra days from the current dating method.
- Enter the completed weeks and extra days reported by ultrasound.
- Review the discrepancy, threshold, recommendation, EDD shift, and dating-quality flag.
How the redating result is calculated
This calculator compares two gestational-age estimates: the age assigned by current dates and the age estimated by ultrasound. It does not calculate gestational age from crown-rump length, biparietal diameter, or any other measurement. The ultrasound age must already come from an appropriate scan and qualified interpretation.
The day counts are:
E = 7 * existingWeeks + existingDays
U = 7 * ultrasoundWeeks + ultrasoundDays
D = U - E is the signed discrepancy, and A = |D| is the absolute discrepancy. If D is positive, the ultrasound estimate is farther along than the current dates, so adopting it would move the EDD earlier. If D is negative, adopting ultrasound dating would move the EDD later.
The ACOG threshold table used here
The threshold comes from the ultrasound gestational age, not from the current dates. The comparison is strict: a discrepancy exactly equal to the threshold does not satisfy the more-than rule.
| Ultrasound gestational age | Redating threshold |
|---|---|
| Before 9 0/7 weeks | More than 5 days |
| 9 0/7 through 15 6/7 weeks | More than 7 days |
| 16 0/7 through 21 6/7 weeks | More than 10 days |
| 22 0/7 through 27 6/7 weeks | More than 14 days |
| 28 0/7 weeks or later | More than 21 days |
For a reliable LMP with no earlier ultrasound, the calculator recommends routine redating only when A is greater than the threshold. For an uncertain LMP with no earlier ultrasound, it favors the earliest accurate ultrasound estimate. When an earlier ultrasound already established the EDD, later discrepancies are not handled as routine replacements. For ART pregnancies, ART-derived dating takes precedence.
What changes the recommendation most
The dating basis often matters as much as the number of days. A six-day difference before 9 weeks can redate a reliable-LMP pregnancy, but the same difference after 9 weeks does not exceed the seven-day threshold. A later scan that conflicts with an earlier accurate ultrasound usually calls for clinical review, not automatic redating.
What this calculator leaves out
The result does not evaluate LMP reliability, cycle length, scan quality, measurement choice, fetal growth restriction, multiple gestation, ART records, or whether the ultrasound is the earliest accurate scan. Third-trimester discrepancies need special caution because abnormal fetal growth can look like a dating difference. The output is a structured summary of the entered dates, not a clinical EDD order.
Worked example
Suppose the current dating method says 8 0/7 weeks and the ultrasound report says 8 6/7 weeks. The current-date count is E = 7 * 8 + 0 = 56 days. The ultrasound count is U = 7 * 8 + 6 = 62 days, so D = 62 - 56 = 6 days and A = 6 days.
Because the ultrasound age is before 9 0/7 weeks, the ACOG threshold is more than 5 days. With a reliable LMP and no earlier ultrasound, 6 days is greater than the threshold, so the calculator recommends using ultrasound dating. Since the ultrasound age is ahead by 6 days, the estimated EDD adjustment is 6 days earlier, and the resulting gestational age is 8 6/7 weeks.
Common questions
How many days must an ultrasound differ before the due date changes?
It depends on the ultrasound gestational age. ACOG uses more than 5 days before 9 0/7 weeks, more than 7 days from 9 0/7 through 15 6/7 weeks, more than 10 days from 16 0/7 through 21 6/7 weeks, more than 14 days from 22 0/7 through 27 6/7 weeks, and more than 21 days at 28 0/7 weeks or later.
Does a discrepancy equal to the ACOG cutoff change the EDD?
No. The threshold is a more-than cutoff. For example, a 7-day discrepancy in the 9 0/7 through 15 6/7 week band does not trigger routine redating by this rule; it would need to be more than 7 days.
Should a later ultrasound replace an early ultrasound due date?
Usually no. Once an early accurate ultrasound has established the EDD, later changes should be reserved for rare circumstances and reviewed clinically. Later fetal size differences may reflect growth patterns rather than wrong dates.
What happens when the last menstrual period is uncertain?
When the LMP is uncertain and there is no earlier ultrasound, the earliest accurate ultrasound estimate generally becomes the preferred dating basis. This calculator therefore recommends ultrasound dating for that setting rather than relying only on the discrepancy table.
How is an IVF or ART pregnancy dated?
ACOG guidance uses the ART-derived gestational age to assign the EDD. For IVF, that means the embryo age and transfer date drive dating. Routine ultrasound redating should not replace ART-derived dating without clinician review.
What does suboptimally dated pregnancy mean?
ACOG describes a pregnancy without an ultrasound examination that confirms or revises the EDD before 22 0/7 weeks as suboptimally dated. This calculator flags that issue when no earlier ultrasound is entered and the ultrasound age is 22 0/7 weeks or later.