Pregnancy Due Date Calculator

About the Pregnancy Due Date Calculator

By convention, the estimated due date (EDD) is 280 days — 40 weeks — after the first day of your last menstrual period (LMP). This calculator works it out from your last period (adjusted if your cycles are longer or shorter than 28 days), a known conception date, an IVF embryo transfer (day 3, 5 or 6) or a first-trimester scan — the crown–rump length (CRL) or the gestational age printed on the report.

It shows how many weeks pregnant you are today, when each trimester starts, the American College of Obstetricians and Gynecologists’ (ACOG) term dates and a week-by-week calendar you can download. If you enter both a last period and a scan, it applies ACOG’s rule for when the scan date should replace the period date. Nothing you enter leaves your browser.

How each method works

• Last period (Naegele’s rule): EDD = LMP + 280 days. The rule assumes a 28-day cycle with ovulation on day 14; with longer or shorter cycles the calculator moves the date by the difference (a 32-day cycle adds 4 days), assuming ovulation is still 14 days before the next period. Naegele’s rule is often taught as “add a year, subtract three months, add seven days”, which can differ from 280 days by a day or two.
• Conception: EDD = conception date + 266 days (280 − 14).
• IVF: ACOG says the due date should come from the embryo’s age and the transfer date — 261 days after transferring a day-5 embryo and 263 days after a day-3 embryo; the same rule gives 260 days for a day-6 embryo.
• First-trimester scan: the Robinson–Fleming formula gives gestational age in days = 8.052 × √CRL (mm) + 23.73, for CRLs up to 84 mm (about 14 weeks). Your scanner’s software may use a slightly different formula, so the gestational age on your report can differ by a day — if you have it, you can enter it directly.

Gestational age is counted from the first day of the LMP (or the date equivalent to it), so you are “2 weeks pregnant” around the time of conception.

When the scan date replaces the period date

ACOG, the American Institute of Ultrasound in Medicine and the Society for Maternal–Fetal Medicine agree that measuring the embryo or fetus in the first trimester (up to 13 6/7 weeks) is the most accurate way to establish the due date — partly because only about half of women recall their LMP accurately. Their table says to change the due date to the ultrasound date when the two differ by more than:

• 5 days for a scan up to 8 6/7 weeks, 7 days from 9 0/7 to 13 6/7 weeks (CRL)
• 7 days from 14 0/7 to 15 6/7 weeks, 10 days from 16 0/7 to 21 6/7 weeks, 14 days from 22 0/7 to 27 6/7 weeks, and 21 days from 28 0/7 weeks (head, abdomen and femur measurements)

The age ranges are counted from the LMP. With an IVF pregnancy the transfer date is used instead. Once a due date has been set from an early scan, ACOG advises against changing it on the basis of a later scan except in rare cases — your doctor or midwife’s date is the one to go by.

Trimesters and term

• First trimester: up to 13 6/7 weeks · second: 14 0/7 to 27 6/7 weeks · third: from 28 0/7 weeks (as used in ACOG Committee Opinion 700).
• Early term 37 0/7 to 38 6/7 weeks · full term 39 0/7 to 40 6/7 weeks · late term 41 0/7 to 41 6/7 weeks · postterm 42 0/7 weeks and beyond (ACOG Committee Opinion 579). Birth before 37 weeks is preterm.

“12 6/7 weeks” means 12 completed weeks and 6 days — the calculator writes it as 12w 6d.

How exact is a due date?

It is an estimate of when 40 weeks will be reached, not a prediction of the day labour starts. ACOG gives the accuracy of ultrasound dating as about ±5–7 days for the CRL up to 13 6/7 weeks, ±7–10 days from 14 0/7 to 21 6/7 weeks, ±10–14 days from 22 0/7 to 27 6/7 weeks and ±21–30 days in the third trimester. A pregnancy without a scan that confirms or revises the due date before 22 0/7 weeks is considered suboptimally dated.

Sources

• ACOG, AIUM and SMFM. Methods for Estimating the Due Date, Committee Opinion 700, including Table 1 “Guidelines for Redating Based on Ultrasonography”
• ACOG and SMFM. Definition of Term Pregnancy, Committee Opinion 579
• Robinson HP, Fleming JE. A critical evaluation of sonar “crown-rump length” measurements. Br J Obstet Gynaecol 82(9):702-10; the formula as used in, for example, AJOG Global Reports 2024 (PMC10998200)

Limitations

• An estimate for planning — your doctor or midwife assigns the due date used for your care.
• The cycle-length adjustment assumes ovulation 14 days before the next period; irregular cycles make last-period dating less reliable.
• The calculation is the same for twins and other multiples; your care team plans the timing of birth for them.
• CRLs of 5–84 mm, scan ages of 4–42 weeks and dates within the last two years are accepted.

Privacy

Everything happens in your browser. What you enter or open here is not uploaded or stored by MySmartCoPilot.