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Pregnancy Due Date Calculator

Estimate your due date from your last period, conception date, ultrasound, or IVF transfer date. See your current gestational week, trimester, and your baby's week-by-week size — instantly, with a clear explanation.

4 due-date methods Weeks 4–40 tracked Free, no sign-up Updated: Jul 22, 2026
Naegele's rule: LMP + 280 days. If your cycle isn't 28 days, the date adjusts automatically.
Calculated as conception date + 266 days — this is usually the day of ovulation.
Enter the gestational age measured at your ultrasound — often the most accurate method, especially with irregular cycles.
Day 3 (cleavage) transfer → due date = transfer + 263 days · Day 5 (blastocyst) transfer → transfer + 261 days.
Quick answer To estimate your due date, add 280 days (40 weeks) to the first day of your last period — this is Naegele's rule. For example, if your last period started March 1, your estimated due date is around December 5. This calculator also supports conception-date, ultrasound, and IVF transfer-date methods, and shows your current week and trimester.
40 wksFull term (Naegele's rule)
280 daysLMP → due date
3Trimesters
~38 wksBaby's actual age at full term
⚕️ Health note: This tool is for informational purposes only and is not a substitute for medical advice. All results are estimates — only about 5% of babies are born exactly on their due date. For your exact dates, measurements, and care plan, please talk to your doctor or midwife.
🤰 Pregnancy Due Date Calculator — ToolPico ↗

What is a pregnancy due date calculator, and how does it work?

A complete guide — with Naegele's rule and real formulas — to estimating your due date, finding out how many weeks pregnant you are, and understanding your trimester.

A pregnancy due date calculator estimates your estimated due date (EDD) and current gestational age from a known reference date. Pregnancy is conventionally counted from the first day of your last menstrual period (LMP), not from the day you actually conceived — mainly because most people know their last period date but not the exact day of conception. That's why, on the calendar, you're already considered "2 weeks pregnant" the day you conceive. The calculator above handles four common dating methods — LMP, conception date, ultrasound, and IVF embryo transfer — instantly and with an explanation.

How is my due date calculated?

Quick answerThe most common method is Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. This is mathematically the same as subtracting 3 months and adding 7 days. If your average cycle is longer or shorter than 28 days, the estimate shifts by the same number of days.
  • Formula: Due date = LMP + 280 days (adjusted for cycle length)
  • Conception-based: Due date = conception date + 266 days
  • Ultrasound-based: Back-dates the LMP-equivalent from the gestational age measured at the scan, then applies the same +280-day rule

How many weeks pregnant am I?

Quick answerYour gestational age is the number of days since the first day of your LMP, divided by 7. If 84 days have passed, you're 12 weeks 0 days pregnant. Enter your dates in the "Last Period (LMP)" tab above to see your exact week and day.

This is the same number your doctor or midwife uses at every prenatal visit. It's also the basis for scheduling routine screenings, since most prenatal tests are recommended within a specific window of weeks (see the reference tables below).

How is a due date calculated after IVF?

Quick answerAfter IVF, the due date is calculated from the embryo transfer date. A day-3 (cleavage-stage) transfer gives an estimated due date of transfer + 263 days; a day-5 (blastocyst) transfer gives transfer + 261 days. The 2-day gap reflects the embryo's age at the moment of transfer.

What's the difference between gestational age and fetal age?

Quick answerGestational age is counted from your last period; fetal (conceptional) age is counted from the estimated date of conception, roughly 2 weeks later. So at "12 weeks pregnant" (gestational age), your baby's actual developmental age is closer to 10 weeks — the baby is always about 2 weeks "younger" than the pregnancy week you're quoted.

What do the three trimesters cover?

Quick answerPregnancy is split into three roughly equal trimesters: first trimester (weeks 1–13), second trimester (weeks 14–27), and third trimester (weeks 28–40). Each trimester lines up with distinct milestones and a different set of recommended prenatal screenings.

Pregnancy month, week & trimester at a glance

Pregnancy lasts 40 weeks and about 9 months — but not every "month" is exactly 4 weeks. Here's the commonly used mapping.

Pregnancy month → week range → trimester
MonthWeeksTrimester
Month 1Weeks 1–41st trimester
Month 2Weeks 5–81st trimester
Month 3Weeks 9–131st trimester
Month 4Weeks 14–172nd trimester
Month 5Weeks 18–222nd trimester
Month 6Weeks 23–272nd trimester
Month 7Weeks 28–313rd trimester
Month 8Weeks 32–353rd trimester
Month 9Weeks 36–403rd trimester

Popular lookups

Week-by-week pregnancy tracker

Browse any week from 4 to 40 to see your baby's estimated size, what's developing, what's happening in your body, and a tip for that week.

Reference tables & practical facts

Citable figures: the prenatal test schedule, IVF due date formulas, and a comparison of dating methods. These are evergreen values that don't change year to year.

Common prenatal screening windows (weeks of pregnancy)
TestWeeksPurpose
NT (nuchal translucency) screening11–14Chromosomal risk screening
Quad screen16–20Chromosomal & neural tube risk
Anatomy scan (Level II ultrasound)20–22Detailed organ & anomaly screening
Glucose tolerance test (OGTT)24–28Gestational diabetes screening
Group B Strep (GBS) culture35–37Bacteria screening before delivery
Non-stress test (NST)36+ (weekly)Fetal well-being check

Your exact schedule may vary — always confirm timing with your own OB/GYN or midwife.

Due date formulas by embryo transfer type
Transfer typeEmbryo age at transferDue date formula
Day 3 (cleavage-stage)3 daysTransfer date + 263 days
Day 5 (blastocyst)5 daysTransfer date + 261 days

The due date is found by subtracting the embryo's lab age from the standard 280-day estimate.

Which dating method should I use?
MethodInputFormulaBest for
Last period (LMP)First day of last periodLMP + 280 daysRegular ~28-day cycles
ConceptionFertilization date+ 266 daysKnown ovulation/conception date
UltrasoundScan date + measured weekBack-dated from scanMost accurate; irregular cycles
IVF transferTransfer dateDay 3: +263 · Day 5: +261IVF pregnancies

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Embed the pregnancy due date calculator on your own website for free. Copy the code below into your HTML — the tool runs in a simplified view and links back to this page as its source.

The embedded tool has a fixed layout; you can adjust the height value to fit your site. No ads or personal data, runs entirely client-side.

Pregnancy terms glossary

Short definitions of the core terms used in due-date calculations.

LMP (last menstrual period)The first day of your last period before pregnancy; the standard starting point for counting gestational age.
Naegele's ruleThe standard method for estimating a due date: LMP + 280 days (equivalent to −3 months, +7 days).
Gestational ageThe age of the pregnancy counted from the LMP, expressed in weeks and days.
Fetal (conceptional) ageThe baby's actual developmental age, counted from conception — about 2 weeks less than gestational age.
Estimated due date (EDD)The predicted delivery date; only about 5% of babies are born exactly on this date.
TrimesterOne of pregnancy's three stages: 1st (weeks 1–13), 2nd (14–27), 3rd (28–40).
CRL (crown-rump length)The head-to-bottom measurement used to date early pregnancies (weeks 4–19) on ultrasound.
Full termReached at 39–40 weeks; babies born from 37 weeks are generally considered "term."

In-depth guides

Step-by-step explanations of the most commonly confused due-date questions.

Step by step: how the due date is calculated

Step 1: Choose your method — LMP, conception date, ultrasound, or IVF transfer.
Step 2: Enter the relevant date (and cycle length, if using LMP).
Step 3: Tap Calculate — the tool applies Naegele's rule (+280 days from an LMP-equivalent date).
Step 4: Review your estimated due date, current week, trimester, days remaining, and baby's estimated size.

Because every method is converted to an "LMP-equivalent" date first, all four tabs use the exact same +280-day math under the hood — only the starting point changes.

Why your due date isn't a deadline

An estimated due date is exactly that — an estimate. Only around 5% of babies are born on their exact due date; most arrive anywhere from 37 to 42 weeks. Because pregnancy is dated from your last period rather than conception, your baby's actual developmental age at "40 weeks pregnant" is closer to 38 weeks — the first two weeks on the calendar happen before conception.

This is also why an ultrasound taken early in pregnancy (ideally in the first trimester) is often considered the most reliable way to confirm or adjust your due date, since it measures the baby directly rather than relying on memory of your last period.

How IVF due dates differ from natural conception

In a natural cycle, gestational age is estimated from the LMP because the exact conception date usually isn't known. In IVF, however, the embryo transfer date is precisely known, along with the embryo's exact age (3 or 5 days) at the moment of transfer — which makes the IVF due-date estimate especially reliable.

The formula simply works backward from the transfer date: a day-5 (blastocyst) embryo is already 5 days old at transfer, so the due date is transfer date + 261 days (280 − 19, since a day-5 embryo corresponds to 19 days of gestational age). A day-3 embryo is 3 days old, giving transfer + 263 days (280 − 17).

Frequently asked questions

How do I calculate how many weeks pregnant I am?
Your gestational age is the number of days since the first day of your last menstrual period (LMP), divided by 7. For example, if 84 days have passed since your LMP, you're 12 weeks 0 days pregnant. Because pregnancy is counted from your last period rather than conception, you're not technically pregnant yet during the first two weeks.
How is a due date calculated?
The most common method is Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period — the same as subtracting 3 months and adding 7 days. This tool can also calculate your due date from a conception date, an ultrasound measurement, or an IVF embryo transfer date.
How many months is 20 weeks pregnant?
20 weeks pregnant is about 5 months. Pregnancy lasts a total of 40 weeks and 9 months, but not every month is exactly 4 weeks. In the commonly used mapping, week 12 falls in month 3, week 20 in month 5, week 28 in month 7, and weeks 36–40 fall in month 9.
How is a due date calculated after IVF?
After IVF, the due date is calculated from the embryo transfer date. For a day-3 (cleavage-stage) transfer, the estimated due date is the transfer date + 263 days; for a day-5 (blastocyst) transfer, it's transfer date + 261 days. The 2-day difference reflects the embryo's age at the time of transfer.
When are prenatal tests done during pregnancy?
NT (nuchal translucency) screening is typically done at weeks 11–14, the quad screen at 16–20, the detailed anatomy ultrasound at 20–22, the glucose tolerance test (OGTT) at 24–28, and the Group B Strep (GBS) culture at 35–37. The non-stress test (NST) is usually done weekly starting around week 36.
At what week can you find out the baby's sex?
Sex can often be estimated by ultrasound around weeks 12–14, with a more reliable result usually confirmed at the detailed anatomy ultrasound between weeks 18 and 22. NIPT blood testing can determine sex with high accuracy starting around week 10.
What is the difference between gestational age and fetal age?
Gestational age is counted from the first day of your last period, while fetal (conceptional) age is counted from the estimated date of conception — usually about 2 weeks later. So at 12 weeks gestational age, your baby's actual developmental age is closer to 10 weeks.
Does this tool store my data?
No. All calculations run entirely in your browser — the dates you enter are never sent to a server. The only way your result becomes shareable is if you tap "Copy link," which encodes your inputs into a URL; sharing that link is entirely up to you.
Can I add my pregnancy milestones to my calendar?
Yes. After calculating your due date, the personalized pregnancy calendar table shows your due date, estimated conception date, trimester start dates, viability (week 24), full term (week 37), and each prenatal test window as real calendar dates. Tap "Download as .ics" to import all of them into Google Calendar, Apple Calendar, or Outlook, or tap "Print" to keep a paper copy.

Methodology & sources

ToolPico's Pregnancy Due Date Calculator is a free, independent tool. The calculation engine is based on Naegele's rule: due date = first day of last menstrual period (LMP) + 280 days (40 weeks), adjusted for cycle length when it differs from 28 days. The conception-date method adds 266 days; the ultrasound method back-dates the LMP-equivalent from the gestational age measured at the scan; and the IVF method back-dates from the embryo transfer date, using a transfer-day embryo age of 17 days (day-3/cleavage transfer) or 19 days (day-5/blastocyst transfer) — equivalent to due date = transfer date + 263 or + 261 days.

Sources: Naegele's rule and standard obstetric dating conventions, consistent with guidance from the American College of Obstetricians and Gynecologists (ACOG); fetal size comparisons follow commonly published week-by-week pregnancy references. These are evergreen figures that don't change from year to year. Last updated: July 22, 2026. Results are estimates for informational purposes only and are not medical advice — only a small percentage of babies are born on their exact due date.

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