Pregnancy Date Calculator

Estimate due date, reverse due date, pregnancy week, gestational age, conception timing, ovulation timing, period timing.

Health estimate

Topic review: Sarah Johansson

Maternal Health Writer. Assigned as the health topic reviewer for pregnancy, fertility, ovulation, and women’s health calculators.

Reviewed 2 May 2026 Updated 5 May 2026 View reviewer profile Contact editorial team

Pregnancy date planning estimate Estimate a due date, reverse due date, pregnancy week, gestational age, conception window, ovulation timing, and pregnancy test timing from the pregnancy date anchor you know best.

Dating method

First day of last period Average cycle length (days) Calculate as of

Estimated due date

18 Dec 2026

Last menstrual period. Use clinician-assigned dates, first-trimester scan dating, or fertility-clinic instructions when those differ from a calendar estimate.

Estimated due date: 18 Dec 2026; 210 days until the estimated due date.
Pregnancy week and gestational age: 10w 0d; First trimester
Estimated conception / ovulation: 27 Mar 2026; Calendar estimate only; real ovulation can move earlier or later.
Reverse due date LMP estimate: 13 Mar 2026; Back-calculated from the estimated due date using the 280-day convention.

Pregnancy week and gestational age

Current gestational age

10w 0d

Trimester

First trimester

Approx fetal age

8w 0d

Reverse due date and conception timing

Timing point	Date	What it means
Last menstrual period	13 Mar 2026	The LMP-equivalent anchor used for gestational dating.
Ovulation estimate	27 Mar 2026	A calendar estimate based on cycle timing, not proof of ovulation.
Conception estimate	27 Mar 2026	Usually close to ovulation, but fertilisation timing cannot be confirmed from dates alone.
Most reliable home pregnancy test date	10 Apr 2026	Usually around the missed-period checkpoint or about 14 days after ovulation.
Estimated due date	18 Dec 2026	A planning anchor, not a guaranteed delivery date.

Conception estimate

Using the first day of your last period, a 28-day cycle, and a 14-day luteal phase, ovulation is estimated around 27 March 2026.

Cycle	Likely conception	Fertile window	Due date	Shift
26-day cycle	25 March 2026	20 March 2026 – 26 March 2026	16 December 2026	2 days earlier
28-day cycle	27 March 2026	22 March 2026 – 28 March 2026	18 December 2026	Selected cycle
30-day cycle	29 March 2026	24 March 2026 – 30 March 2026	20 December 2026	2 days later
32-day cycle	31 March 2026	26 March 2026 – 1 April 2026	22 December 2026	4 days later
35-day cycle	3 April 2026	29 March 2026 – 4 April 2026	25 December 2026	7 days later

Ovulation estimate and period timing

Estimated ovulation is 27 Mar 2026; the broader fertile window is 22 Mar 2026 to 28 Mar 2026, and the next period estimate is 10 Apr 2026.

Pregnancy test timing

Late follow-up window The main home-test window has passed. A persisting missed period with negative tests should be reviewed with a clinician.

Window	Timing	Date	Meaning
Implantation window opens	6 DPO	2 Apr 2026	Pregnancy-related hCG is not expected in urine before implantation begins.
Sensitive early-test window	10 DPO	6 Apr 2026	Some early-result tests may detect hCG around 10 DPO, but false negatives are still common.
Most reliable home-test date	14 DPO	10 Apr 2026	This is the first-day-of-missed-period benchmark most guidance treats as the main home-testing window.
Retest if negative and still no period	16 DPO	12 Apr 2026	If the first test was negative, a repeat in 48 hours improves confidence when bleeding still has not started.

Planning milestones

End of first trimester: 18 Jun 2026

20-week mark: 31 Jul 2026

37-week mark: 27 Nov 2026

39-week mark: 11 Dec 2026

41-week mark: 25 Dec 2026

42-week mark: 1 Jan 2027

Medical disclaimer This calculator is for pregnancy planning and education only. It does not diagnose pregnancy, confirm paternity, assess symptoms, replace IVF-specific dating, or override care from a midwife, obstetrician, GP, fertility clinic, or emergency service.

About this calculator

Health estimate Reviewed 2 May 2026 Updated 5 May 2026

Topic review

Sarah Johansson

Maternal Health Writer. Assigned as the health topic reviewer for pregnancy, fertility, ovulation, and women’s health calculators.

Formula provenance

The methodology summary explains the formula family in use, and the source list points to the primary or authoritative references checked for this topic.

Methodology

Uses existing Calcipedia pregnancy dating logic for due date, reverse due date, pregnancy weeks, gestational age, conception, ovulation, and pregnancy test timing. LMP mode applies the 280-day gestational convention with cycle-length adjustment for due-date and ovulation context. Conception mode projects 266 days forward. Known-due-date mode back-calculates an LMP-equivalent date. Dating-scan mode projects from gestational age at scan to the 40-week point.

Limitations

Calendar-based pregnancy dating can be inaccurate when the last period is uncertain, cycles are irregular, ovulation timing differs from the assumption, or an ultrasound has revised the estimated due date.
Conception, ovulation, and fertile-window estimates are orientation aids only and cannot confirm fertilisation, implantation, paternity, or pregnancy viability.
The page does not replace IVF-specific embryo-transfer dating, hCG trend interpretation, ultrasound interpretation, symptom assessment, or clinician-led pregnancy care.
Pregnancy test timing depends on the test, hormone levels, cycle timing, and the instructions supplied with the specific test.

Disclaimer

Use this calculator for general pregnancy planning and education only. It does not diagnose pregnancy, assess symptoms, confirm conception, replace emergency care, or override advice from your midwife, obstetrician, GP, sonographer, fertility clinic, or another qualified clinician.

Primary health sources

NHS — Due date calculator — Public NHS due-date guidance using last menstrual period and average cycle length.
ACOG — When Pregnancy Goes Past Your Due Date — Patient guidance explaining that pregnancy is commonly dated from the first day of the last menstrual period and that due dates are estimates.
NHS — Doing a pregnancy test — Public guidance on when pregnancy tests can be taken and how to interpret timing around a missed period.
Mayo Clinic — Home pregnancy tests — Patient guidance on home pregnancy test timing, false negatives, and practical testing considerations.
MedlinePlus — Pregnancy test — Consumer health information on pregnancy test types, hCG detection, and result interpretation.
NICE — Ectopic pregnancy and miscarriage: diagnosis and initial management — Clinical guideline relevant to safety routing for pain, bleeding, and early-pregnancy concerns.

Change notes

Health change note: the formula, reference ranges, source set, disclaimer, and safety guidance for this calculator were last reviewed on 2026-05-02. Report a correction at contact@calcipedia.org; substantive source or guidance changes refresh both the reviewed date and the updated date shown on the page.

See our calculation methodology and editorial standards, plus the editorial contact route, for how this estimate is produced and corrected.

← All Fertility & Pregnancy calculators

Pregnancy Dating

Pregnancy date calculator: due date, weeks pregnant, conception, ovulation

A pregnancy date calculator brings several closely related pregnancy timing questions into one workflow. Use it to estimate a due date, reverse due date, pregnancy week, gestational age, conception timing, ovulation timing, period timing, and home pregnancy test timing from the pregnancy date anchor you know best: last menstrual period, estimated conception date, known due date, or dating scan details.

What this pregnancy date calculator is designed to consolidate

People rarely search for pregnancy timing in a single neat sequence. One person may start with a last period date and want an estimated due date. Another may already have a due date from a scan report and want to know how many weeks pregnant they are. Someone else may be working backwards from a due date to understand a rough conception window, ovulation estimate, period timing, or when a pregnancy test was most likely to become reliable.

This page connects those related questions without removing the more focused calculators. The due date calculator, pregnancy weeks calculator, reverse due date calculator, gestational age calculator, conception calculator, ovulation calculator, period calculator, and pregnancy test calculator still serve their own search intent. This master page is a broader pregnancy date calculator for users who need the whole timing picture in one place.

The four dating methods available

Last-period mode starts with the first day of the last menstrual period and applies the standard 40-week pregnancy dating convention, with an average-cycle-length adjustment for due date and ovulation context. Conception mode starts from an estimated conception date and projects forward using the usual relationship between conception timing and gestational dating.

Known-due-date mode works backwards from an estimated due date already given in an app, scan report, referral letter, or maternity record. Dating-scan mode starts from the scan date and the gestational age measured at that scan, then projects forward to the 40-week point.

Estimated due date = LMP + 280 days + (cycle length - 28 days)

The last-period path uses the standard 40-week convention and adjusts the due-date estimate when the average cycle is not 28 days.

Estimated due date = conception date + 266 days

The conception path uses the common approximation that conception is about 266 days before the estimated due date.

LMP equivalent = known due date - 280 days

The reverse due date path back-calculates the gestational dating anchor from an existing estimated due date.

Estimated due date = scan date + (280 days - gestational age at scan)

The dating-scan path projects from the gestational age already assigned at the scan.

Due date, pregnancy week, and gestational age in one result

The headline due date is useful, but it is only part of pregnancy dating. The result also shows current pregnancy week and gestational age in weeks and days, trimester, approximate fetal age, and key milestones such as the 20-week mark, 37-week mark, 39-week mark, 41-week mark, and 42-week mark.

That makes the page useful for searches such as pregnancy date calculator, pregnancy due date calculator, pregnancy weeks calculator, how many weeks pregnant am I, and gestational age calculator. The same result can answer today's question while giving enough context for appointments, scan timing, and date comparisons.

Reverse due date and conception timing

Reverse due date calculations work backwards from an estimated due date to an LMP-equivalent date and an estimated conception date. This can help you understand the timeline implied by a due date, but it should not be treated as proof of the exact day conception happened.

Conception and ovulation are biological events with natural variation. A calendar estimate can be useful for orientation, but it cannot confirm ovulation, fertilisation, paternity, implantation, or pregnancy viability.

Ovulation, period timing, and pregnancy test timing

The calculator also estimates ovulation timing, fertile-window context, next-period timing, and home pregnancy test timing where those values can be inferred from the selected dating method. This helps users who are trying to connect an LMP date, missed period, ovulation estimate, and pregnancy test result without jumping between separate tools.

Home pregnancy tests are usually more reliable after the missed-period checkpoint or around two weeks after ovulation. Testing too early can give a false negative because hormone levels may not yet be high enough for the test to detect.

What remains separate and why

Some pregnancy and fertility topics should not be collapsed into a general pregnancy date calculator. IVF due date calculations depend on embryo-transfer date and embryo age. hCG level interpretation depends on measured blood results, timing, trend, and clinical context. Fertility-by-age estimates and broader fertility planning answer different questions from pregnancy dating.

For that reason, this page links related tools but does not replace specialist pages. Keeping those pages separate protects long-tail keywords, avoids flattening medically sensitive guidance, and helps users choose the calculator that matches the information they actually have.

Worked example: known due date converted into the full timeline

Suppose a scan report lists an estimated due date of 8 October 2026 and you want to understand the pregnancy timeline on 2 April 2026. Known-due-date mode works backwards 280 days to an LMP-equivalent date of 1 January 2026, then counts forward to the as-of date. The result is 13 weeks and 0 days pregnant on 2 April 2026.

The same result estimates conception around 15 January 2026, shows the pregnancy as first trimester under the calculator's boundaries, and places the 37-week milestone on 17 September 2026. If a clinician has assigned a different date, use the clinician-assigned estimate for care decisions.

Medical limitations

This page is for planning and education only. It does not diagnose pregnancy, confirm paternity, assess bleeding or pain, interpret ultrasound findings, replace IVF-specific dating, or override dates assigned by a midwife, obstetrician, GP, sonographer, fertility clinic, or emergency service.

Seek urgent medical advice if you are pregnant or may be pregnant and have severe pain, heavy bleeding, fainting, shoulder-tip pain, fever, or other concerning symptoms. Calendar tools cannot rule out ectopic pregnancy, miscarriage, complications, or the need for urgent assessment.

Frequently asked questions

What is the best pregnancy date calculator mode to use?

Use last-period mode if you know the first day of the last menstrual period. Use known-due-date mode if a clinician or scan report has already given you an estimated due date. Use dating-scan mode if you know the scan date and gestational age at scan. Use conception mode only when the conception date is a reasonable estimate, not a guess.

Is a pregnancy date calculator the same as a due date calculator?

A due date calculator focuses on the estimated due date. This pregnancy date calculator includes the due date but also shows reverse due date, pregnancy week, gestational age, conception estimate, ovulation timing, period timing, pregnancy test timing, and milestone dates.

Can I calculate how many weeks pregnant I am from a due date?

Yes. Enter the known estimated due date and the as-of date. The calculator works backwards to an LMP-equivalent date, then counts forward to show pregnancy weeks and days, trimester, and milestone timing.

Can this calculator tell the exact conception date?

No. It can estimate a likely conception date or window from standard pregnancy dating assumptions, but it cannot prove exactly when ovulation, fertilisation, implantation, or conception occurred.

Why does the calculator show gestational age as more than fetal age?

Gestational age is counted from the first day of the last menstrual period. Fetal or embryonic age is usually approximated from conception, which is often about two weeks later in a 28-day cycle. That is why gestational age is usually about two weeks higher.

When should I take a pregnancy test?

Home pregnancy tests are generally more reliable after a missed period or around two weeks after ovulation. Testing earlier may produce a false negative, so follow the test instructions and repeat or seek medical advice if the result and symptoms do not match.

Does cycle length change my pregnancy dates?

Cycle length can shift an LMP-based due date and ovulation estimate because ovulation often happens later in longer cycles and earlier in shorter cycles. It does not replace ultrasound dating or a clinician-assigned estimated due date.

Should I use this page for IVF pregnancy dating?

No. IVF and embryo-transfer dating use transfer date and embryo age. Use an IVF due date calculator or the dates provided by your fertility clinic.

Why were the older pregnancy calculators not redirected into this page?

They cover distinct search intent and medically sensitive long-tail questions. They remain live until a medical review and production keyword data show that redirecting them would not remove useful content, weaken keyword coverage, or confuse users.

Can this calculator replace advice from my maternity team?

No. Use it as a planning aid. For scan discrepancies, irregular cycles, symptoms, fertility treatment, medication decisions, or any concern about pregnancy health, follow guidance from a qualified clinician.