Pregnancy Due Date Calculator

Estimate your due date from your last period, conception date or IVF transfer, and see how many weeks along you are, your trimester and the key dates ahead.

Calculate from
The day your last period started (day 1 of bleeding).
days
From one period’s first day to the next. Leave at 28 if unsure.

Your dates stay on this device. They are not saved, added to the page address or sent anywhere.

Example Enter your own date above.

Estimated due date

Tuesday, February 9, 2027

40 weeks from the first day of your last period (May 5, 2026). It is an estimate: most babies arrive in the weeks around it, not on the day.

How far along today
20 weeks, 3 days
20w 3d · day 143 of about 280
Trimester
Second
14w 0d to 27w 6d
Days to go
137
about 19 weeks, 4 days
Likely conception
Tue, May 19, 2026
Estimate: about 2 weeks after the period start

Key dates

Key dates for a due date of February 9, 2027
MilestoneDate(s)Today
First trimesterEnds at 13 weeks, 6 daysPassedMay 5 – Aug 10, 2026Passed
Second trimester14w 0d to 27w 6dNowAug 11 – Nov 16, 2026Now
Anatomy ultrasoundUsually done at 18–22 weeksNowSep 8 – Oct 12, 2026Now
Gestational diabetes screeningUsually at 24–28 weeksOct 20 – Nov 23, 2026
Tdap (whooping cough) vaccineRecommended at 27–36 weeks, earlier in the window preferredNov 10, 2026 – Jan 18, 2027
Third trimesterFrom 28w 0d until birthFrom Nov 17, 2026
Group B strep test36w 0d to 37w 6dJan 12 – Jan 25, 2027
Early term37w 0d to 38w 6dJan 19 – Feb 1, 2027
Full term39w 0d to 40w 6dFeb 2 – Feb 15, 2027
Estimated due date40w 0dTue, Feb 9, 2027
Late term41w 0d to 41w 6dFeb 16 – Feb 22, 2027
Postterm42w 0d and beyondFrom Feb 23, 2027

Working

  1. First day of your last menstrual period (LMP): Tuesday, May 5, 2026.
  2. Naegele’s rule: LMP + 280 days (40 weeks) = Tuesday, February 9, 2027. The rule assumes a 28-day cycle with ovulation around day 14.
  3. Your cycle is 28 days, so no adjustment is needed.
  4. Today, September 25, 2026, is 143 days after May 5, 2026: 143 ÷ 7 = 20 weeks, 3 days (written 20w 3d).
  5. Trimester: 20w 3d is in the second trimester (first: up to 13w 6d; second: 14w 0d to 27w 6d; third: from 28w 0d).
  6. Days to go: from September 25, 2026 to February 9, 2027 = 137 days.

An estimate for information only, not medical advice. If your clinician has dated your pregnancy by ultrasound, their due date takes precedence. This calculator cannot tell how a pregnancy is progressing; contact your prenatal care provider with any concerns.

How to use the due date calculator

Pick how you want to calculate: from the first day of your last period (the usual method), from a known conception date, or from an IVF embryo transfer. Enter the date, and for the period method adjust the cycle length if your cycles are regularly longer or shorter than 28 days. The due date, how far along you are today, your trimester and a table of key dates appear straight away, with the working underneath. Until you change anything, the example shows a pregnancy that is 20 weeks, 3 days along. Everything is worked out in your browser, and the dates are never stored or sent.

How the due date is calculated

ACOG, the American College of Obstetricians and Gynecologists, dates pregnancy from the first day of the last menstrual period (LMP). By convention the due date is 280 days, or 40 weeks, later. The other methods convert to the same scale:

From last period: due date = LMP + 280 days + (cycle length − 28) days From conception: due date = conception + 266 days From IVF transfer: due date = transfer + 263 days (day-3 embryo) due date = transfer + 261 days (day-5 embryo) Weeks pregnant today = (today − (due date − 280 days)) ÷ 7

The 280-day rule assumes ovulation on day 14 of a 28-day cycle. With longer cycles, ovulation usually comes later, so the cycle adjustment moves the due date (and the week count) by the difference. It is a common refinement, but it relies on regular cycles; with irregular cycles an early ultrasound is the dependable way to date a pregnancy. All the arithmetic here is done on calendar dates, so leap years and month lengths are handled exactly, and the result never shifts with time zones.

Worked example

Suppose a last period started on Monday, March 2, 2026, and cycles are usually 31 days.

  • Naegele’s rule: March 2, 2026 + 280 days = December 7, 2026.
  • The cycle is 3 days longer than 28, so the estimated due date moves to Thursday, December 10, 2026.
  • Weeks are counted from the adjusted start, March 5, 2026. On September 25, 2026 that is 204 days, or 29 weeks, 1 day (29w 1d): the third trimester, with 76 days to go.
  • Full term (39 weeks 0 days to 40 weeks 6 days) runs from December 3, 2026 to December 16, 2026.

Enter March 2, 2026 with a 31-day cycle to see the same due date; the week count will reflect today’s date.

A due date is an estimate

Pregnancies vary naturally in length. In a 2013 study, researchers at the NIH’s National Institute of Environmental Health Sciences found that among pregnancies where the day of ovulation was known, length still varied by as much as 37 days. Only about 4% of births came exactly 280 days after the last period, and about 70% came within 10 days of the estimated due date. That is why it helps to think of a due window rather than a single day.

Ultrasound dating takes precedence. ACOG’s Committee Opinion 700 says that measuring the embryo or fetus by ultrasound in the first trimester is the most accurate way to set the due date. If your clinician’s date differs from this calculator’s, use theirs. For IVF pregnancies, ACOG recommends dating from the embryo’s age and the transfer date, which is what the IVF option does.

Trimesters, term and the key dates

ACOG counts the first trimester up to 13 weeks 6 days, the second from 14 weeks 0 days to 27 weeks 6 days, and the third from 28 weeks 0 days. Clinicians write gestational age as weeks and days, so “20w 3d” (or 20 3/7) means 20 whole weeks and 3 days. The key dates table uses these published windows:

  • Anatomy ultrasound: the standard ultrasound exam is usually done at 18–22 weeks (ACOG).
  • Gestational diabetes screening: usually at 24–28 weeks, or earlier if you have risk factors (ACOG).
  • Tdap vaccine: the CDC recommends it during weeks 27 through 36 of every pregnancy, ideally early in that window, to help protect the newborn from whooping cough.
  • Group B strep test: ACOG recommends screening between 36 weeks 0 days and 37 weeks 6 days.
  • Term: early term 37w 0d–38w 6d, full term 39w 0d–40w 6d, late term 41w 0d–41w 6d, postterm from 42w 0d (ACOG Committee Opinion 579). Birth before 37 weeks is preterm.

Your own care schedule may differ, depending on your health and history. To count the days between any two dates, use the date difference calculator; the age calculator works out exact ages in years, months and days once your baby arrives.

Frequently asked questions

How is a due date calculated?

The standard method, known as Naegele’s rule, adds 280 days (40 weeks) to the first day of your last menstrual period. It assumes a 28-day cycle with ovulation around day 14, so if your cycles are usually longer or shorter, the date is often moved by the difference. From a known conception date, add 266 days; from an IVF transfer, add 263 days for a day-3 embryo or 261 days for a day-5 embryo.

How accurate is my due date?

It is a best estimate, not a deadline. In a 2013 study by NIH researchers, only about 4% of births came exactly 280 days after the last period, and only about 70% came within 10 days of the estimated due date, even with ultrasound dating. Most babies arrive in the weeks around the date rather than on it.

Why does the calculator say I was 2 weeks pregnant when I conceived?

Pregnancy is dated from the first day of the last period, about two weeks before ovulation and conception, because that date is usually known and conception usually is not. So at conception the gestational age is already about 2 weeks, and a “40-week” pregnancy lasts about 38 weeks from conception.

My ultrasound gave a different due date. Which one should I use?

Use the one your clinician gives you. ACOG says an ultrasound measurement in the first trimester (up to 13 weeks 6 days) is the most accurate way to date a pregnancy, and the due date is changed to the ultrasound date if the two differ by more than 5 days before 9 weeks or more than 7 days from 9 weeks to 13 weeks 6 days, with wider margins for scans later in pregnancy.

How do I calculate an IVF due date?

Add 261 days to the transfer date for a day-5 embryo (blastocyst) or 263 days for a day-3 embryo. That is 266 days from fertilization minus the embryo’s age at transfer. ACOG recommends dating IVF pregnancies this way, from the embryo’s age and the transfer date, rather than from a period. Your fertility clinic will confirm the date.

What do early term, full term and late term mean?

They are ACOG’s labels for when a baby is born: early term is 37 weeks 0 days to 38 weeks 6 days, full term 39 weeks 0 days to 40 weeks 6 days, late term 41 weeks 0 days to 41 weeks 6 days, and postterm 42 weeks or later. Birth before 37 weeks is preterm.

Sources

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