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Due Date Calculator: How to Work Out Exactly How Far Along You Are
Take the first day of your last period and add 280 days. That is the whole trick behind every due date calculator, and it is the same arithmetic your doctor does at the first visit. Pregnancy is counted as about 40 weeks from the last menstrual period, according to the NICHD, and 40 weeks is 280 days. Everything else on this page is about when that simple sum gets adjusted, and what the date actually promises (less than you'd think).
Why the count starts before you were pregnant
Clinicians date pregnancy from the last menstrual period (LMP), not from the day the egg was fertilized. MedlinePlus puts it plainly: during weeks 1 and 2 of pregnancy you are not pregnant yet; ovulation, and the moment you are most likely to conceive, comes at the end of week 2. So by the time you reach "40 weeks," it has been about 38 weeks since conception.
Why use a date that is two weeks off? Because you can point to it on a calendar. Most people know roughly when their last period started. Almost nobody knows the hour an egg was fertilized.
Two formulas fall out of this:
- Estimated due date = first day of your last period + 280 days.
- Weeks pregnant = days since your last period started ÷ 7. Eighty-four days after your period began, you are 12 weeks.
Worked example: a period that started on January 1 gives a due date of October 8 in a non-leap year. Many Janeen users in the Gulf also want the Hijri date; the app converts it for you, but for the calculation itself always use the Gregorian date your clinic writes down, so the two records match.
The four ways a due date gets set
From your last period
The default, and the one above. The NICHD notes that providers calculate the due date "based on your last menstrual cycle and ultrasound exam," which is a polite way of saying the period date gets a second opinion. If your cycles are irregular, or you genuinely cannot recall the date, say so at the first appointment; the scan will carry more weight.
From a known conception date
If you tracked ovulation, subtract the two "phantom" weeks: conception date + 266 days. The conception-date guide walks through fetal age versus gestational age with examples.
From an early ultrasound
MedlinePlus lists "determine the baby's age" as one of the jobs of a first-trimester ultrasound, done in the first 12 weeks. In a study of 208 singleton IVF pregnancies published in Ultrasound in Obstetrics & Gynecology (Tunón and colleagues, 2000), where the true conception date was known, the first-trimester crown-rump measurement differed from it by an average of only 0.9 days. That is why an early scan can override a period date, and why a later growth scan usually does not: babies grow at different speeds after the first trimester, so size becomes a worse clock.
After IVF
The most exact of all, because nothing is guessed. The CDC's assisted-reproduction glossary explains that for IVF cycles the "LMP" is estimated from the date of egg retrieval or transfer, and the Tunón study used retrieval + 14 days as the equivalent of the period date. Work it through and you get: due date = egg retrieval + 266 days, which is a day-5 transfer + 261 days, or a day-3 transfer + 263 days.
One line from MedlinePlus to keep in mind: the due date is calculated from the first day of your last period, the size of your uterus early in pregnancy, and an early ultrasound. Three inputs, one estimate.
Janeen calculates your due date with all four methods, then walks with you week by week — baby's size, length and weight, kick counter, symptom diary and smart checkup reminders. Free, and your data never leaves your phone.
Download Janeen freeWhat the date actually means
MedlinePlus says it in four words: a due date is an estimate. Most pregnancies last 37 to 42 weeks. The NICHD splits that stretch into named bands, and the names matter because the 37th and 38th weeks used to be called "term" and no longer are.
Source for the table: NICHD, "Know Your Terms."
| Birth at | NICHD term |
|---|---|
| Before 37 weeks | Preterm (34–36 weeks is "late preterm") |
| 37 weeks – 38 weeks 6 days | Early term |
| 39 weeks – 40 weeks 6 days | Full term |
| 41 weeks – 41 weeks 6 days | Late term |
| 42 weeks and beyond | Post-term |
The NICHD adds that if there is no medical reason to deliver earlier, it is best to deliver at or after 39 weeks; between 37 and 39 weeks the brain, lungs and liver are still finishing critical development. So a due date is really the middle of a window, not an appointment. Pack the hospital bag around week 36 and let the baby choose.
If the date comes and goes, MedlinePlus describes what happens next: at about 41 weeks your provider checks on the baby with a non-stress test and an ultrasound biophysical profile, and between 41 and 42 weeks will likely want to induce labor, because the risks rise past 42 weeks. Only a small number of pregnancies get that far.
Questions people type into the app
How do I calculate my due date from my last period?
Add 280 days (40 weeks) to the first day of your last menstrual period. The NICHD describes pregnancy as lasting about 40 weeks measured from the last menstrual period, and your provider will compare that date with an early ultrasound.
How accurate is a due date calculator?
The formula is exact; the inputs are not. MedlinePlus calls the due date an estimate built from the last period date, early uterus size and an early ultrasound, and says most pregnancies last 37 to 42 weeks.
How do I calculate my due date after IVF?
Count from egg retrieval: retrieval + 266 days. That works out to 261 days after a day-5 embryo transfer or 263 days after a day-3 transfer, because the CDC treats retrieval or transfer as the stand-in for the last period in IVF cycles.
How many weeks pregnant am I?
Count the days from the first day of your last period until today and divide by 7. Eighty-four days after your period started, you are 12 weeks pregnant, which is also the end of the first trimester by the NICHD's week 1–12 definition.
Can my due date change?
Yes, usually once and early. If a first-trimester ultrasound disagrees with your period date, your provider may adopt the scan date. Later scans mostly check growth rather than reset the calendar.
The bottom line
Start with 280 days from your last period, then let your care team confirm it against the earliest scan or your IVF dates. Keep that one agreed date in Janeen, use the prenatal appointment guide to plan the visits ahead, and follow along in the week-by-week guide.
Official sources
- NICHD: About Pregnancy (length of pregnancy, trimesters, term definitions) — accessed September 8, 2026.
- NICHD: Know Your Terms (early, full, late and post-term) — accessed September 8, 2026.
- NICHD: What happens during prenatal visits? — accessed September 8, 2026.
- MedlinePlus: When you pass your due date — accessed September 8, 2026.
- MedlinePlus: Fetal development (gestational age from the last period) — accessed September 8, 2026.
- MedlinePlus: Pregnancy ultrasound — accessed September 8, 2026.
- Office on Women's Health: Prenatal care and tests — accessed September 8, 2026.
- CDC: ART glossary (gestational age in IVF cycles) — accessed September 8, 2026.
- Tunón K et al., Ultrasound Obstet Gynecol 2000 (PubMed): gestational age after IVF vs crown-rump length — accessed September 8, 2026.