Pregnancy Due Date Calculator Widget

Add a pregnancy due date calculator to your website. Visitors choose the first day of their last period, a conception date or an IVF transfer date and see the estimated due date, how many weeks and days along they are, the trimester and the key dates ahead.

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<iframe src="https://a2z.tools/embed/w/pregnancy-due-date-calculator" title="Pregnancy Due Date Calculator by A2Z Tools" width="100%" height="800" style="border:0;width:100%" loading="lazy" allow="clipboard-write"></iframe>

A plain iframe. Works everywhere, including site builders that strip scripts. Adjust height if your content needs more room.

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How it works

From the last menstrual period the widget applies the convention ACOG describes - the due date is 280 days after the first day of the last period, which assumes a regular 28-day cycle with ovulation on day 14 - and shifts the date by the difference when the visitor's usual cycle is longer or shorter, so a 32-day cycle moves it four days later. A known conception date adds 266 days, and IVF transfers use the embryo age, as ACOG recommends for assisted reproduction: 263 days for a 3-day embryo and 261 days for a 5-day blastocyst. Gestational age is counted from the equivalent period date and shown as weeks plus days with the trimester. The key-dates table uses ACOG's term definitions (early term from 37 weeks, full term from 39) and adds 42 weeks. Arithmetic is done on whole calendar days, so leap years and daylight-saving changes cannot shift a date.

Calculation method

  • From LMP: due date = LMP + 280 days + (cycle length - 28 days)
  • From conception: due date = conception + 266 days
  • From IVF transfer: 3-day embryo + 263 days; 5-day embryo + 261 days (266 - embryo age)
  • Gestational age = today - (due date - 280 days), shown as whole weeks + days
  • Trimesters: 1st to 13 weeks 6 days, 2nd from 14 weeks, 3rd from 28 weeks; ACOG: early term 37+0 to 38+6, full term 39+0 to 40+6, late term 41 weeks, post-term 42 weeks

Worked examples

Period on 1 January 2026, 28-day cycle

Inputs: LMP 2026-01-01, cycle 28 days, checked on 2026-03-12

Result: Due date 8 October 2026; 10 weeks 0 days; first trimester

1 January + 280 days = 8 October; 12 March is 70 days after the LMP.

Same period, 32-day cycle

Inputs: LMP 2026-01-01, cycle 32 days

Result: Due date 12 October 2026

Four days later than the 28-day rule, for the later ovulation.

An estimate for general information only - not medical advice. Your midwife or doctor will confirm your dates, usually with an ultrasound scan.

Limitations

  • Assumes a regular cycle and an accurately remembered period date; irregular cycles, recent hormonal contraception or breastfeeding make the LMP method unreliable.
  • Does not use scan measurements, so it cannot re-date a pregnancy the way a first-trimester ultrasound does.
  • The trimester boundaries are the common 14- and 28-week convention; services in some countries count them slightly differently.

Where publishers use it

  • Parenting, pregnancy and baby-product websites
  • Midwife, doula and antenatal-class sites where clients check their dates before a first appointment
  • Fertility clinics and IVF information pages, using the transfer-date options
  • Maternity clothing and nursery shops that plan content around due dates

Questions

How accurate is a due date?

It is an estimate. ACOG considers a first-trimester ultrasound (crown-rump length, accurate to about 5-7 days) the most accurate way to establish or confirm the date, and the scan date then takes precedence over any date-based rule.

Why does cycle length change the date?

The 280-day rule assumes ovulation on day 14 of a 28-day cycle. With a longer or shorter cycle ovulation usually happens later or earlier, so the widget moves the date by the difference. This adjustment is a common calculator convention, not part of the ACOG rule.

Why do IVF transfers use 263 and 261 days?

A 3-day embryo transfer corresponds to conception three days earlier and a 5-day transfer to five days earlier, so the transfer date plus 266 minus the embryo age gives the due date.

Does the widget store the date?

No. The date is only used in the visitor's browser to draw the result.

Sources

  1. Committee Opinion No. 700: Methods for estimating the due date - American College of Obstetricians and Gynecologists, 2017 . 280 days from LMP; ultrasound dating; ART dates from embryo age and transfer date
  2. Committee Opinion No. 579: Definition of term pregnancy - American College of Obstetricians and Gynecologists, 2013 . Early, full, late and post-term boundaries

Cite or recommend this tool

If you reference this tool in an article, course or documentation, these formats are ready to copy. They are optional - nothing is added to your site unless you paste it.

A2Z Tools Pregnancy Due Date Calculator
https://a2z.tools/pregnancy-due-date-calculator

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