Enter the gestational age on the date of your ultrasound or checkup, and we will work out your last period, your current week and your estimated due date.
If your ultrasound report gives a gestational age such as “9 weeks 4 days”, counting back that many days from the scan date gives your last menstrual period by dates. Adding 280 days gives the estimated due date, and comparing with today gives your current week (UpToDate, 2025). If the time to today is 42 weeks 0 days or more, the tool shows a postterm note; if the calculated last period falls in the future or the time exceeds 45 weeks, it asks you to check the reference date.
Example: if a scan on 10 March 2026 shows 9 weeks 4 days, the tool counts back 9 × 7 + 4 = 67 days. The last period by dates is 2 January 2026 and the estimated due date is 9 October 2026.
If you have more than one report, use the earliest one. Crown-rump length (CRL) measured in the first trimester is the most accurate measure of gestational age. The due date confirmed or set by the earliest ultrasound done at or before 21 weeks 6 days becomes your due date and is not changed by later scans (UpToDate, 2025).
Head, abdomen and thigh-bone measurements taken in the second and third trimesters are used to check whether the baby's growth is in the normal range, not to change dates that have already been set. That is why the “ultrasound age” on a later report can differ from your first scan; dating follows the first scan.
The tool finds the start date that matches the baby's measured size, which may not be the date on your calendar. You may have ovulated after day 14, your cycle may be longer than 28 days, bleeding mistaken for a period may have had another cause, or you may have conceived after stopping contraception before your first period; the baby's development may also not be progressing as expected.
In one study the median difference between ultrasound-based and period-based gestational age was 1 day (interquartile range −1 to 5 days); because late ovulation is more common than early ovulation, ultrasound more often moves the due date later. However, if in the first trimester the baby measures clearly smaller than your dates, this is not always explained by late ovulation; your doctor may reassess the pregnancy's development after a short interval (UpToDate, 2025).
If the difference on a first-trimester scan is larger than expected for the week of measurement, the due date is set by ultrasound; if it is within that limit, the period-based date is kept (UpToDate, 2025). Your doctor decides which date to use. Enter that date into the due date calculator or the pregnancy calendar to see your whole timeline.
The tool also works with the following information:
| What you know | Reference date | Weeks + days to enter |
|---|---|---|
| Estimated due date | Due date | 40 weeks 0 days |
| Fresh IVF cycle | Egg retrieval date | 2 weeks 0 days |
| Frozen day-3 embryo transfer | Transfer date | 2 weeks 3 days |
| Frozen day-5 embryo (blastocyst) transfer | Transfer date | 2 weeks 5 days |
These values follow from the due date being 280 days after the last period, 266 days after egg retrieval, 263 days after a day-3 transfer and 261 days after a day-5 transfer (UpToDate, 2025).
This tool does not make a diagnosis; your doctor confirms your gestational age and due date.
Count back from the scan date by the gestational age measured that day: weeks × 7 + days. For 9 weeks 4 days, that is 67 days. The tool does this for you and also gives your due date.
Count back 280 days from the estimated due date. In the tool, enter your due date as the reference date, 40 for weeks and 0 for days.
The earliest one, ideally from the first trimester. A due date set by the earliest ultrasound done at or before 21 weeks 6 days is not changed by later scans (UpToDate, 2025).
Enter the transfer date as the reference date, with 2 weeks 3 days for a day-3 embryo or 2 weeks 5 days for a day-5 embryo. In a fresh cycle, the egg retrieval date counts as 2 weeks 0 days.
The tool counts back from the gestational age that matches the baby's measured size. Late ovulation, a long cycle or bleeding mistaken for a period can explain the difference. If the difference is larger than expected for the week of measurement, the due date is set by ultrasound; your doctor decides which date to use.
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