Pregnancy due date calculator
Pick the date you know best. The calculator gives your estimated due date, how far along you are today, and your dates for the main prenatal tests, using the dating rules in ACOG Committee Opinion 700.
1. What date do you know?
2. Enter it
The day bleeding started, not the day it ended.
Day 1 of one period to day 1 of the next.
Use a positive ovulation-test day or an IUI date if you have one.
Fresh or frozen transfer: the rule is the same.
Your clinic's transfer note lists this.
Reports often write this as "GA 8w2d" or "CRL = 8+2".
Add it to see whether the gap between the two dates is big enough that ACOG's table supports switching to the scan date.
On This Page – Quick Medical Summary
A due date is set 280 days, or 40 weeks, after the first day of the last menstrual period, the convention in ACOG Committee Opinion 700, Methods for Estimating the Due Date (2017). The calculator also accepts the three other starting points that opinion recognizes: a conception or ovulation date, an IVF embryo transfer, and the age measured on an ultrasound.
Whichever one you use, you get your estimated due date, how far along you are today, a window of 10 days on each side, and calendar dates for the main prenatal milestones. If you have an early scan as well as a period date, enter both under Ultrasound, and the calculator checks the gap against ACOG’s redating table.
How each method counts to your due date
Every method looks for the same starting point, the day a pregnancy counts as 0 weeks 0 days, and adds 280 days to it. What changes is the evidence used to find that day.
| What you know | How the due date is found | Worked example |
|---|---|---|
| First day of last period | Period date + 280 days | May 1 → Feb 5 |
| Conception or ovulation date | That date + 266 days | May 15 → Feb 5 |
| IVF transfer, day-5 embryo | Transfer date + 261 days | May 20 → Feb 5 |
| IVF transfer, day-3 embryo | Transfer date + 263 days | May 18 → Feb 5 |
| Ultrasound | Scan date + (280 − age at scan in days) | Scan July 1 at 8 weeks 5 days → Feb 5 |
Source: ACOG Committee Opinion 700 for the 280-day, day-5 and day-3 rules. The 266-day figure is 280 days minus the 14 days the period rule assumes before ovulation. All five rows describe one sample pregnancy.
The period rule assumes a 28-day cycle with ovulation on day 14. That means it can’t allow for a cycle that runs longer or shorter, or for a start date remembered wrongly. Only about half of women recall that date accurately. With a 35-day cycle, ovulation probably came about a week later than the rule expects, so the calculator moves every date a week later to match. Your clinician may keep the unadjusted date until an early scan settles it.
Months differ in length, which trips up the familiar shortcut of adding 7 days and subtracting 3 months. A May 1 period gives February 8 by the shortcut and February 5 by counting 280 days.
Conception dating works best with a tracked ovulation date or an insemination date. For IVF, the opinion sets the due date from the embryo’s age and the transfer date, and the period plays no part.
Can an ultrasound change your due date?
ACOG’s 2017 opinion calls an ultrasound measurement in the first trimester, up to 13 6/7 weeks, the most accurate way to set or confirm gestational age. ACOG writes ages as weeks plus sevenths, so 13 6/7 means 13 weeks 6 days. At that stage, the crown-rump length (the straight-line length from head to bottom) dates a pregnancy to within about 5 to 7 days.
That precision moves a lot of due dates. In a randomized trial cited in the opinion, 40% of the women given a first-trimester scan had their due date adjusted. In each case, scan dating and LMP dating differed by more than 5 days.
ACOG uses a single cutoff for each stage. When the gap is larger than the cutoff, the scan date replaces the LMP date.
| Age by period date at the scan | Switch to the scan date if the gap is more than |
|---|---|
| Up to 8 weeks 6 days | 5 days |
| 9 weeks 0 days – 13 weeks 6 days | 7 days |
| 14 weeks 0 days – 15 weeks 6 days | 7 days |
| 16 weeks 0 days – 21 weeks 6 days | 10 days |
| 22 weeks 0 days – 27 weeks 6 days | 14 days |
| 28 weeks 0 days or later | 21 days |
Source: ACOG Committee Opinion 700, Table 1.
Take a period that began on May 1, which makes the pregnancy 8 weeks 5 days on July 1. Suppose a scan that day measures 7 weeks 2 days, which is 10 days behind. The cutoff at that stage is 5 days, so the calculator switches to the scan’s due date, February 15, and explains the switch in a box under the result. If the scan had measured 8 weeks 2 days, a gap of 3 days, the period-based date of February 5 would stand.
Once an early scan has set the date, the opinion keeps later changes for rare circumstances. A pregnancy with no scan confirming or revising its due date before 22 weeks 0 days counts as suboptimally dated.
4% of babies arrive on the due date
A 2013 cohort study measured ovulation with daily urine hormone tests in 125 natural pregnancies. The time from ovulation to birth averaged 268 days, yet it still varied by as much as 37 days (ESHRE summary of the Human Reproduction paper). The same summary reports that only 4% of women deliver at 280 days and only 70% within 10 days of their due date, even when ultrasound helped set it.
Two patterns stood out. Each additional year of maternal age added roughly one day to the pregnancy, and women with longer pregnancies before tended to have longer ones again.
That spread is why the result shows a window of 10 days either side of the estimate.
The weeks after 37 also carry different labels. In 2013, ACOG and the Society for Maternal-Fetal Medicine replaced the single word “term” with four categories, because outcomes differ across them.
| Label | Weeks of pregnancy |
|---|---|
| Early term | 37 weeks 0 days – 38 weeks 6 days |
| Full term | 39 weeks 0 days – 40 weeks 6 days |
| Late term | 41 weeks 0 days – 41 weeks 6 days |
| Postterm | 42 weeks 0 days and later |
Source: ACOG Committee Opinion 579, Definition of Term Pregnancy (2013).
The term-status box in the calculator uses these labels.
Trimester and test dates on your results
US dating counts the first trimester through 13 weeks 6 days, the second from 14 weeks 0 days to 27 weeks 6 days, and the third from 28 weeks 0 days. These are the ranges in ACOG’s redating table. Calculators that end the first trimester at 12 weeks show trimester dates a week earlier than this one.
Under the summary, the results table turns weeks into calendar dates. It covers the anatomy ultrasound, glucose screening, the group B strep swab and each term milestone, and grays out anything already past.
Glucose screening in pregnancy often begins with a sugary drink and one blood draw an hour later. The guide to what the 1-hour glucose test checks and what happens after a high result covers the details.
Later ultrasounds report growth against the same week count. The fetal growth percentile calculator compares a measurement with others at the same week. Weight-gain targets follow those weeks too in the pregnancy weight gain calculator.
41 weeks and still pregnant
A 2024 American Family Physician review of ACOG Practice Bulletin 146 (2014) summarizes ACOG’s guidance. From 41 weeks 0 days, ACOG recommends fetal monitoring, and by 42 weeks it recommends induction of labor for everyone. The reason is the rise in stillbirth across the term weeks. A 2019 meta-analysis of cohort studies covering about 15 million pregnancies found 0.11 stillbirths per 1,000 births at 37 weeks and 3.18 per 1,000 at 42 weeks.
Very few pregnancies get that far now. The same review cites recent US data showing 0.25% of pregnancies continuing to 42 weeks, compared with 5.5% reported in 2014.
For a low-risk first pregnancy, ACOG also allows an elective induction from 39 weeks 0 days. In the large randomized trial behind that option, induction lowered cesarean births (one fewer cesarean for every 28 people induced) without a rise in problems for the baby. Ask your doctor in the final weeks whether monitoring from 41 weeks or an induction at 39 weeks fits your pregnancy.
Get emergency care now if: at any stage of pregnancy your baby moves less or stops moving, you have vaginal bleeding or fluid leaking, or you have a headache that won’t go away or gets worse. The same applies to vision changes, severe belly pain that doesn’t go away, trouble breathing, chest pain, a fever of 100.4°F or higher, or extreme swelling of your hands or face. All of these are on the CDC’s list of urgent maternal warning signs. Call 911 or go to the emergency room and say you are pregnant. If you have thoughts of harming yourself or your baby, call or text 988.
Bring these dates to your first prenatal visit
Take the result screen or the downloaded CSV file to your first prenatal appointment. That’s where your care team sets the official due date, from your period date, an early ultrasound or both. Questions worth raising there:
- Will my due date come from my period date or from an ultrasound, and when will that scan be?
- My cycles usually run longer or shorter than 28 days. Does that change how you date this pregnancy?
- In which weeks are my anatomy scan, glucose test and group B strep swab?
- If I reach 41 weeks, what monitoring do you offer, and when would we talk about induction?
If you’re still trying to conceive, the ovulation calculator estimates your fertile days. A tracked ovulation date can then go straight into the conception method above.
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About this content
How this article was put together: researched from recognised health sources, drafted with the help of AI tools, and edited by hand, with sources linked throughout.
Dr. Pooja Sahu, MBBS, is a medical reviewer for MyMedicineAdvisor. She is a clinical physician whose experience includes serving as a Junior Resident in Nephrology at Dr. Ram…
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The content on My Medicine Advisor is provided for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Health information on this website should not be used to diagnose, treat, cure, or prevent any condition without guidance from a qualified healthcare professional. Always seek the advice of your doctor, physician, or another licensed healthcare provider with any questions you may have regarding a medical condition, symptoms, medications, or treatment decisions.
