Dating Pregnancy Calculator
Dating pregnancy calculator works out How far along and Clinical notation from Date to report on, First day of the last menstrual period and Estimated due date. The default figures on this page give how far along of 21w 4d; change any entry and the result, the working and the sources update together.
Your figures
Updates as you typeHow this is worked out
7 steps
- Ovulation is one luteal phase before the next period would start: 28 - 14 = day 14 of the cycle, so 2026-01-15.
- Estimated due date = ovulation + 266 days.
- With a 28-day cycle this is exactly Naegele's rule: LMP + 280 days.
- Gestational age on 2026-06-01 = 2026-06-01 - 2026-01-01 = 151 days.
- 151 days = 21 completed weeks and 4 day(s), written 21 4/7 weeks.
- Fetal (conceptional) age is 14 days less: 137 days.
- Days to the due date = 280 - 151 = 129.
Next milestonetable
| label | gestationalAge | date |
|---|---|---|
| Limit of periviability | 24 0/7 weeks | 2026-06-18 |
Its 5 worked examples are re-run on every release, last on 15 September 2026. What changed
Sources and references
3 sources
What the dating pregnancy calculator calculates
Reports how many weeks and days pregnant someone is on a given date, with trimester, fetal age, days remaining and the next dating milestone, from the LMP, a due date, a conception date, an ultrasound or an IVF transfer.
How to use this dating pregnancy calculator
Enter Date to report on, First day of the last menstrual period, Estimated due date and Conception or ovulation date. How far along updates as you type, and “Use a worked example” loads checked figures you can adjust. Change one figure at a time to see what drives the result.
How the result is worked out
Open “How this is worked out” beside the result for the step-by-step calculation behind How far along, Clinical notation and Completed weeks.
The method follows ACOG Committee Opinion No. 700: Methods for Estimating the Due Date, ACOG Committee Opinion No. 579: Definition of Term Pregnancy and WHO, Preterm birth fact sheet (definition of preterm as before 37 completed weeks).
21 weeks 4 days on 1 June 2026 from a 1 January 2026 LMP example
With What do you know?: lmp, First day of the last menstrual period: 2026-01-01, Date to report on: 2026-06-01, the calculator returns Gestational age in days: 151, Completed weeks: 21, Extra days: 4. This example is checked against an independent source in the test suite.
What you can compare
Change one input at a time to see how it affects How far along, Clinical notation and Completed weeks. The inputs you can vary are Date to report on, First day of the last menstrual period, Estimated due date, Conception or ovulation date, Date of the ultrasound and Gestational age at the scan (weeks).
Understanding How far along, Clinical notation and Completed weeks
An estimate from calendar arithmetic, not a clinical assessment. Only about 4% of births occur on the estimated due date; ultrasound dating by a clinician supersedes menstrual dating when they disagree.
Results are general information, not financial, tax, legal, medical or engineering advice. Check any decision that matters with a qualified professional.
See something wrong? Report a problem with this calculator
Frequently asked questions
How far pregnant will I be on a certain date?
On the figures this page opens with, the how far along is 21w 4d. No single number fits everyone: it follows Date to report on, First day of the last menstrual period and Estimated due date, so enter your own above and the answer updates as you type.
Is 4 weeks pregnant actually 2 weeks?
Reports how many weeks and days pregnant someone is on a given date, with trimester, fetal age, days remaining and the next dating milestone, from the LMP, a due date, a conception date, an ultrasound or an IVF transfer. Put your own figures in above and the working updates beside the result. For scale, the default figures on this page give 21w 4d.
How far pregnant am I if Clearblue says 2-3 weeks?
It needs Date to report on, First day of the last menstrual period, Estimated due date, Conception or ovulation date, Date of the ultrasound and Gestational age at the scan (weeks). Nothing else is required, and the figures stay in your browser.
Can I calculate my pregnancy date based on my conception date?
Reports how many weeks and days pregnant someone is on a given date, with trimester, fetal age, days remaining and the next dating milestone, from the LMP, a due date, a conception date, an ultrasound or an IVF transfer. Open “How this is worked out” beside the result to see each step for your own figures.
What information do I need for the dating pregnancy calculator?
You need Date to report on, First day of the last menstrual period, Estimated due date, Conception or ovulation date, Date of the ultrasound, Gestational age at the scan (weeks), Gestational age at the scan (extra days) and Embryo transfer date. The page opens with default figures that give how far along of 21w 4d, so you can see the scale before entering your own.
What does this dating pregnancy calculator show?
It shows How far along, Clinical notation, Completed weeks, Extra days, Gestational age in days, Trimester, Trimester range and Fetal (conceptional) age, with the working and any warnings for your entries.