Pregnancy Due Date & Ovulation Calculator
Calculate your estimated due date, ovulation day, fertile window, and current gestational age from your last menstrual period or estimated conception date, adjusted for your cycle length.
What to Expect by Trimester
| Gestational Age | Trimester | General Overview |
|---|---|---|
| Weeks 0-13 | First Trimester | Morning sickness is common as the baby's major organs begin forming. Miscarriage risk is relatively higher during this period, so paying attention to how you feel is often recommended. |
| Weeks 14-27 | Second Trimester | Morning sickness usually eases and energy often improves. You may start feeling fetal movement, and many ultrasounds during this stage can reveal the baby's sex. |
| Week 28+ | Third Trimester | The belly grows noticeably and physical strain increases. This stage is typically used to prepare for birth, with checkups often becoming more frequent. |
About Due Date Calculators
The standard obstetric method for estimating a due date is "Naegele's rule," which adds 280 days to the first day of your last menstrual period (LMP). This tool goes a step further by factoring in your actual cycle length to estimate your conception and ovulation dates, then adds 266 days from conception to reach the due date — giving a more accurate estimate for cycles shorter or longer than the typical 28 days.
Alongside the due date, you can check your fertile window (roughly 5 days before ovulation through the day after), your current gestational age, and which trimester you are in. All of these are statistical averages, and it is common for the actual delivery date to fall within about two weeks before or after the estimated due date.
How to Use
- Choose a calculation method Select whether to calculate from your last menstrual period or from an estimated conception date (such as an IVF transfer date or ovulation test result).
- Enter the base date Pick the corresponding date on the calendar — either your last menstrual period start date or your estimated conception date.
- Adjust your cycle length (LMP method only) If your average cycle differs from 28 days, entering your actual cycle length improves the accuracy of the estimate.
- Review your results Your estimated due date, current gestational age, ovulation day, and fertile window are all displayed together.
Tips for getting more out of it
- If your cycle is irregular, enter the average cycle length from the past few months for a more realistic estimate of your ovulation day and due date.
- If you know your estimated conception date (such as an IVF transfer date or an ovulation test result), choosing "from estimated conception date" tends to be more accurate than calculating from the last menstrual period.
- Very few pregnancies deliver exactly on the estimated due date — it is common to deliver anywhere within about two weeks before or after. Treat the due date as a general estimate rather than a fixed deadline.
- If your doctor has confirmed a due date based on an ultrasound measurement, that estimate is generally more accurate and should take priority over this calculator's result.
Common Uses
Getting an early estimate after a positive test
Get a rough sense of your due date and gestational age before your first prenatal appointment confirms an official due date.
Planning conception timing
Use your cycle length to estimate ovulation and your fertile window as a reference when trying to conceive.
Planning maternity leave or travel
Use the estimated due date as an early reference point when discussing maternity leave dates or travel plans with your employer.
Previewing upcoming prenatal checkups
See which trimester you are entering next based on your current gestational age, so you know what kind of checkups to expect.
Glossary
- Cycle Length
- The number of days from the start of one period to the day before the next one begins. The typical average is 28 days, but it varies widely between individuals, commonly ranging from 20 to 45 days.
- Gestational Age
- The standard obstetric way of measuring pregnancy duration, counted from the first day of the last menstrual period (LMP) — not the conception date — as "week 0, day 0."
- Ovulation
- The day an egg is released from the ovary, typically estimated to occur about 14 days before the next expected period, since the luteal phase length is relatively consistent.
- Fertile Window
- The days around ovulation when conception is most likely, accounting for sperm survival (several days) and the egg's fertilizable window (about 24 hours after ovulation).
- Last Menstrual Period (LMP)
- The date your most recent period began. It is the standard reference date used in most pregnancy calculations, including Naegele's rule.
- Naegele's Rule
- A traditional formula, named after German obstetrician Franz Naegele, that estimates the due date by adding 280 days to the last menstrual period date.
Frequently Asked Questions
Side Note — The History of Naegele's Rule
The common method of estimating a due date by adding 280 days to the last menstrual period is known as "Naegele's rule," named after 19th-century German obstetrician Franz Karl Naegele. Interestingly, historical research suggests Naegele may not have been the sole originator of the formula, which appears to have emerged from the combined observations of several obstetricians of the era. Even so, it remains the standard reference used in obstetric practice around the world today.
The rule assumes a 28-day cycle with ovulation occurring exactly at the midpoint, on day 14. In reality, cycle lengths vary considerably between individuals, and cycles shorter or longer than 28 days are quite common. This tool includes a cycle length input specifically to correct for that assumption and produce a more realistic due date estimate.
There is also a practical reason gestational age is counted from the last menstrual period rather than the conception date: pinpointing the exact day of conception is often difficult, while the start of the last period is comparatively easy to recall. This convention leads to a somewhat counterintuitive result — at "2 weeks pregnant," conception typically has not yet actually taken place.