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Dr Pixel 2.0Women's imaging diagnosis

PLATFORM FOR EDUCATION AND DECISION SUPPORT IN WOMEN'S IMAGING

Gestational Age and Estimated Due Date Calculator

You can type the date (MM/DD/YYYY) or pick it from the calendar.

After entering the ultrasound date, provide the gestational age (weeks and days) reported at that exam.

MM/DD/YYYY
August 2026
SMTWTFS
Understand the method

How pregnancy dating works

Gestational age can be estimated by two methods: early ultrasound (crown–rump length — CRL) or LMP (last menstrual period). The two methods do not have the same precision, and the choice between them changes how reliable the estimated due date (EDD) is.

The same date, two different confidence widths

Margin of error by method

Early ultrasound± 3–5 days
LMP± 7 days
-7d-5d-3d0+3d+5d+7d
estimated date

Early ultrasound (1st trimester)

  • Ideal window: between 7 weeks and 13 weeks + 6 days, by crown–rump length (CRL).
  • Expected error: ± 3–5 days — the smallest margin across the entire pregnancy.

In this early window embryonic growth is very uniform between fetuses, and genetic and environmental factors have not yet meaningfully affected size — which is why CRL is the most accurate standalone variable for dating, outperforming LMP alone, BPD, HC, AC, or FL. ISUOG and WHO guidelines recommend defining gestational age primarily by first-trimester ultrasound.

Practical rule: once gestational age is set by first-trimester CRL, it should not be revised by later scans, even if subsequent biometry does not match that date.

LMP (Last Menstrual Period)

LMP-based dating assumes ovulation occurred 14 days after the start of the menstrual cycle, with fertilization shortly thereafter — gestational age is counted from the first day of the last menstrual period.

LMP is reliable for dating only when all of the conditions below are met:

  • the date is known and remembered with confidence, with no rough estimate;
  • menstrual cycles are regular, usually between 24 and 32 days;
  • there are no recent factors that alter ovulation (recent hormonal contraception use or discontinuation, breastfeeding, PCOS, significant stress, major weight change, thyroid dysfunction);
  • the reported bleeding matches a usual menstrual period, not spotting or implantation bleeding.

Expected error: even under ideal conditions, LMP has an average error of ± 7 days.

In practice, early ultrasound is the preferred method whenever available; LMP is a useful initial clinical estimate, but not the gold standard for dating.

Common questions

Frequently asked questions about pregnancy dating

What is the difference between dating a pregnancy by ultrasound and by LMP?+
Early ultrasound (1st trimester) has an expected error of ± 3–5 days, while LMP has an average error of ± 7 days even under ideal conditions. ISUOG and WHO guidelines recommend defining gestational age primarily by first-trimester ultrasound whenever it is available.
Until when can early ultrasound be used to date a pregnancy?+
The ideal window for ultrasound dating is between 7 weeks and 13 weeks + 6 days, by crown–rump length (CRL). In this window embryonic growth is most uniform between fetuses, keeping the dating error to a minimum.
Once gestational age is set by first-trimester ultrasound, should it be corrected by later scans?+
No. The best gestational age is the one defined earliest. Once it is set by first-trimester CRL, it should not be revised by biometry from later scans, even if those measurements do not exactly match the original date.
When can LMP be safely used to date a pregnancy?+
LMP is reliable only when the date is known with confidence, menstrual cycles are regular (usually between 24 and 32 days), there are no recent factors altering ovulation (such as hormonal contraception use or discontinuation, breastfeeding, or PCOS), and the reported bleeding matches a usual period, not spotting.
What is the EDD (estimated due date) and how is it calculated?+
The EDD is the estimated delivery date, calculated from 40 weeks of pregnancy. When dating is by ultrasound, the EDD is the exam date plus (40 weeks − gestational age at the ultrasound). When dating is by LMP, the EDD is the LMP plus 280 days (40 weeks).
Is the estimated probable conception date the same as the LMP?+
No. The estimated probable conception date is calculated as approximately 14 days after the LMP (assuming ovulation at that point in the cycle), not the last menstrual period date itself.