Important: Not medical advice. This report is a general estimate from a published formula. It is not medical advice, a diagnosis, or a treatment plan, and it doesn't account for your health history, medications, pregnancy or other conditions. Talk to a doctor, midwife or registered dietitian before making health decisions based on it. Use of this report is at your own risk; see aiocalc.com/disclaimer and aiocalc.com/terms.

Time & Date

Due Date Calculator

Estimate a pregnancy due date from your last period, conception, or IVF transfer, or use the extra tabs to count days between dates or add and subtract days.

Updated October 11, 2026 · 2 cited sources

Your dates

Due DateBetweenAdd/Subtract

Estimated due date

—

Enter a date to see your estimate.

How far along

—

Trimester —

Estimated conception

—

LMP: —

Where you are in the pregnancy

Show the math

Estimate only — actual delivery dates vary widely, and this calculator isn't a substitute for dating confirmed by your care provider, especially via early ultrasound.

How your due date is calculated

The standard method (Naegele's rule) adds 280 days — 40 weeks — to the first day of your last menstrual period (LMP). If you know your conception date or IVF transfer date instead, this calculator works out the equivalent LMP first (conception is typically 14 days after LMP; IVF transfers happen 17 or 19 days after, for day-3 and day-5 transfers respectively), then applies the same 280-day rule from there.

Worked example: an LMP of January 1, 2026 plus 280 days gives October 8, 2026. A conception date of January 15 gives the same answer (January 15 + 266 days), and so does a day-5 transfer on January 20 (+ 261 days). For general date arithmetic outside pregnancy, the date calculator counts days between any two dates.

Adjusting for cycle length

Naegele's rule assumes a 28-day cycle with ovulation around day 14. If your cycles are usually longer, you probably ovulated later, so the pregnancy is a little younger than the raw LMP date suggests; if they're shorter, it's a little older. The calculator moves the due date and the estimated conception date by your cycle length minus 28 days: a 32-day cycle pushes an October 8 due date to October 12, and a 25-day cycle pulls it to October 5.

The adjustment only applies when you enter an LMP, and it accepts cycles from 21 to45 days. If your cycles vary a lot from month to month, an early ultrasound will date the pregnancy more reliably than any period-based formula. The ovulation calculator uses the same cycle-length logic to estimate a fertile window.

How far along am I?

Gestational age is measured in weeks and days from your LMP, not from conception — which is why "6 weeks pregnant" means about 4 weeks since conception. This calculator reports both your current gestational age and trimester (1st: through 13 weeks 6 days, 2nd: 14w0d to 27w6d, 3rd: 28 weeks onward) based on today's date, or any date you choose. For a week-by-week view with pregnancy-test timing, see the pregnancy calculator.

When did I conceive?

Conception is estimated as 14 days after the start of your last menstrual period, assuming a typical 28-day cycle — this calculator applies that same estimate whether you entered your LMP, due date, or IVF transfer date, since all of them anchor back to the same underlying LMP-based timeline. With an LMP and a different cycle length, the conception estimate moves by the same number of days as the due date.

What to expect by trimester

The first trimester (through 13 weeks 6 days) covers early development and the highest miscarriage risk window; the second (14w0d to 27w6d) is often called the most comfortable stretch; the third (28 weeks onward) is when the baby gains most of its birth weight and delivery preparation begins. Only about 5% of babies are born exactly on their due date — most arrive within about 2 weeks of it in either direction.

Trimester week ranges
Trimester	Gestational age	Highlights
First trimester	0w0d – 13w6d	Organs form; highest miscarriage-risk window
Second trimester	14w0d – 27w6d	Often the most comfortable stretch; anatomy scan around 18–22 weeks
Third trimester	28w0d to birth	Most weight gain for the baby; full term is 39w0d – 40w6d

Sources

The formulas on this page come from the published research below. Last updated October 11, 2026. This calculator gives estimates for general information. It isn't medical advice, so check anything important with a qualified professional.

American College of Obstetricians and Gynecologists. Committee Opinion No. 700: Methods for Estimating the Due Date. Obstet Gynecol. 2017;129(5):e150–e154.Used for: 280-day (40-week) due date from the last menstrual period, and IVF transfer-date adjustments
American College of Obstetricians and Gynecologists. Committee Opinion No. 579: Definition of Term Pregnancy. Obstet Gynecol. 2013;122(5):1139–1140.Used for: Early, full, late and post-term ranges around the due date

Frequently asked questions

Can my due date change after an ultrasound?

Yes. ACOG's guidance is to change the due date if a first-trimester ultrasound disagrees with the period-based date by more than 5 days before 9 weeks, or by more than 7 days between 9w0d and 13w6d. Later in pregnancy the allowed gap widens, because babies vary more in size. Once your provider sets a date, it normally stays fixed.

What if I don't remember the first day of my last period?

Use another date you know, such as a conception or IVF transfer date, by changing the "I know my" option. If you have none, an early ultrasound is the standard way to date the pregnancy: measuring the embryo in the first trimester is the most accurate method available. Until then, treat any estimate as rough.

How many weeks is a full-term pregnancy?

ACOG and the Society for Maternal-Fetal Medicine split it up: early term is 37w0d to 38w6d, full term 39w0d to 40w6d, late term 41w0d to 41w6d, and postterm 42w0d or later. The 40-week due date sits inside the full-term window, which is why providers talk about a window of weeks rather than one day.

Does the due date count from conception or from my period?

From the first day of your last period. That convention means the first two weeks of the 40 happen before you're actually pregnant, so at 8 weeks of gestational age the embryo is about 6 weeks old. Ultrasound reports and prenatal appointments all use the period-based count, which is what this calculator shows.

How is my due date calculated after IVF or a frozen embryo transfer (FET)?

Select IVF transfer date (Day 3 or Day 5) above instead of LMP — this calculator works out the equivalent LMP first (a day-3 transfer is 17 days after LMP, a day-5 transfer is 19 days, since embryos are transferred at that developmental age) and applies the same 280-day rule from there. This is more precise than an LMP-based estimate since the transfer date is known exactly, unlike ovulation timing.