Dating from the last period assumes a regular 28-day cycle, so it can be off by a week or more if your cycle is longer or shorter. Ultrasound dating corrects it. Pregnancies conceived through IVF or with known ovulation use different starting points.
This is an educational tool, not medical advice.
Enter the first day of your last menstrual period and the date you want to check. Gestational age is counted from that first day, which is about two weeks before conception, so a woman who is 20 weeks pregnant conceived about 18 weeks ago. The tool shows completed weeks plus extra days, and the due date at 280 days.
If your provider has dated the pregnancy by ultrasound, use that adjusted date instead.
Pregnancy Week: worked example
| First day of last period | May 1, 2026 |
|---|---|
| As of date | September 24, 2026 |
| Completed weeks | 20 |
| Extra days | 6 |
| Estimated due date | February 5, 2027 |
Pregnancy Week: formula
Common error: Counting from conception. That gives a number two weeks lower than the one your care team uses, so milestones and appointments will not line up.
Pregnancy Week: changing First day of last period
| First day of last period | Completed weeks | Extra days | Estimated due date |
|---|---|---|---|
| April 17, 2026 | 22 | 6 | January 22, 2027 |
| April 24, 2026 | 21 | 6 | January 29, 2027 |
| May 1, 2026 (example) | 20 | 6 | February 5, 2027 |
| May 8, 2026 | 19 | 6 | February 12, 2027 |
| May 15, 2026 | 18 | 6 | February 19, 2027 |
Pregnancy Week: compare input ranges
| Input | Tested from | To | Completed weeks: low input | Completed weeks: high input | Swing |
|---|---|---|---|---|---|
| First day of last period | April 24, 2026 | May 8, 2026 | 21 | 19 | 2 (10%) |
| As of date | September 17, 2026 | October 1, 2026 | 19 | 21 | 2 (10%) |
Pregnancy Week: individual input comparisons
| Input | Value | Completed weeks | Extra days | Estimated due date |
|---|---|---|---|---|
| First day of last period | April 17, 2026 | 22 | 6 | January 22, 2027 |
| April 24, 2026 | 21 | 6 | January 29, 2027 | |
| May 1, 2026 | 20 | 6 | February 5, 2027 | |
| May 8, 2026 | 19 | 6 | February 12, 2027 | |
| May 15, 2026 | 18 | 6 | February 19, 2027 | |
| As of date | September 10, 2026 | 18 | 6 | February 5, 2027 |
| September 17, 2026 | 19 | 6 | February 5, 2027 | |
| September 24, 2026 | 20 | 6 | February 5, 2027 | |
| October 1, 2026 | 21 | 6 | February 5, 2027 | |
| October 8, 2026 | 22 | 6 | February 5, 2027 |
Pregnancy Week: combined input changes
| First day of last period / As of date | September 10, 2026 | September 24, 2026 | October 8, 2026 |
|---|---|---|---|
| April 17, 2026 | 20 | 22 | 24 |
| April 24, 2026 | 19 | 21 | 23 |
| May 1, 2026 | 18 | 20 | 22 |
| May 8, 2026 | 17 | 19 | 21 |
| May 15, 2026 | 16 | 18 | 20 |
Pregnancy Week: which assumptions move the result?
Pregnancy Week: values and definitions
| Quantity and definition | Value used |
|---|---|
| First day of last period | May 1, 2026 |
| As of date — Defaults to today. | September 24, 2026 |
| Completed weeks | 20 |
| Extra days | 6 |
| Estimated due date | February 5, 2027 |
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Pregnancy weeks are counted from the first day of the last period, about two weeks before conception. So at the moment of conception you are already counted as two weeks pregnant. A period that started on May 1 makes late September week 20 plus a few days.
The same count runs to a due date at 40 weeks, which is when a clinician expects, not promises, delivery.
Confirm the date is the first day of bleeding, not the last. A wrong start date shifts every week and the due date with it.
Estimate the due date on the due date calculator, and see the conception date and IVF due date pages for other starting points. The pregnancy weight gain calculator uses the week to give the expected gain to date. The period and ovulation calculators cover cycle planning.
Should I replace a provider’s assigned due date with this result?
No. This arithmetic estimates gestational age from an entered menstrual date. A clinician can use ultrasound, treatment dates and other information to assign the pregnancy’s dating.
Does the output count completed weeks?
Yes. Twenty weeks and six days means 146 elapsed days since the entered starting date, not 21 completed weeks.
Why are weeks of pregnancy counted from the last period?
Because the last period is usually known, while conception is not, so doctors count 40 weeks from that date, about two weeks before conception.
What are the trimesters?
The first trimester runs to the end of week 13, the second from week 14 to week 27 and the third from week 28 until birth.
What if my dates and my ultrasound disagree?
A clinician reconciles menstrual dates, ultrasound findings and other pregnancy information using the applicable dating guidance. The discrepancy thresholds depend on gestational age. Do not replace an established clinical due date with a calendar result merely because the two differ.