Due Date Calculator - Pregnancy Week by Week

Estimate your due date from your last menstrual period or conception date, with current gestational age, trimester, and key milestones.

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Select a date to see your estimated due date.

Due Date Calculator Using Naegele's Rule

Find your estimated due date in seconds from either the first day of your last menstrual period or, if you know it, the conception date. The calculator applies Naegele's rule, adding 280 days (40 weeks) to the LMP, or 266 days when counting from conception. Alongside the due date it shows how far along you are today in weeks and days, which trimester you are in, and calendar dates for the 12, 20, 28, and 37 week milestones.

Expecting parents use it the moment a test turns positive, before the first midwife or doctor appointment confirms dates by ultrasound. It is also handy for planning: booking the anatomy scan around 20 weeks, arranging parental leave near the 37-week full-term mark, and knowing when the third trimester begins for travel and insurance rules.

Naegele's rule dates back to the early 1800s and remains the standard first estimate used by obstetric care worldwide. It assumes a 28-day cycle with ovulation on day 14, which is why counting from conception subtracts exactly two weeks. Pregnancy is conventionally measured in gestational weeks from the LMP, so at the moment of conception you are already considered 2 weeks pregnant.

Keep expectations realistic: only about 4-5% of babies arrive on their exact due date, and most are born within a two-week window either side. A first-trimester ultrasound can shift the estimate, and cycle lengths that differ from 28 days move ovulation and therefore the true date. This is an estimate only, consult your midwife or doctor for medical decisions.

Trying to conceive or tracking cycles? The Ovulation Calculator on ToolForte estimates your fertile window and ovulation day across the next three cycles using the same calendar method midwives describe, and the Date Calculator adds or subtracts days from any date, which is handy for counting toward appointments, scan windows, or the start of leave. Both tools are free, run entirely in your browser, and never store any of your health information on a server.

How the Due Date Calculator Works

  1. 01Choose your starting point: last menstrual period or conception date.
  2. 02Select the date in the date picker.
  3. 03The calculator adds 280 days to the LMP (Naegele's rule) or 266 days to conception.
  4. 04Read your due date, current gestational age, trimester, and milestone dates.

Understanding Your Estimated Due Date

Pregnancy dating starts from the first day of the last menstrual period, not from conception, which is why you are considered 4 weeks pregnant when a period is just one week late. Naegele's rule adds 280 days (40 weeks) to the LMP, assuming a 28-day cycle with ovulation on day 14; if you know the conception date, adding 266 days lands on the same due date. The calculator turns this into a practical timeline: gestational age today in weeks and days, your current trimester, and dates for the 12-week end of the first trimester, the 20-week anatomy scan window, the 28-week start of the third trimester, and 37 weeks when the pregnancy is considered full term. Two caveats keep expectations realistic. First, a dating ultrasound in the first trimester frequently adjusts the estimate by a few days and takes precedence. Second, cycles longer or shorter than 28 days shift ovulation, so the calendar estimate drifts accordingly. This is an estimate only; consult your midwife or doctor for care decisions.

When to Use the Due Date Calculator

Use it right after a positive test to get a first timeline before your booking appointment, when planning around milestones such as the anatomy scan or the start of parental leave, and when you want to know your exact gestational age in weeks and days for symptom guides and apps. The conception-date mode suits IVF and tracked cycles where the fertilization date is known precisely.

Common Use Cases

  • Get a first due date estimate right after a positive test.
  • Check your gestational age in weeks and days for a pregnancy app.
  • Plan the 20-week anatomy scan and tell work before the third trimester.
  • Date an IVF pregnancy precisely from the known conception date.
  • Count days to any milestone with a general date tool. → Date Calculator

Expert Tips

  • Write down the LMP date you used; every appointment will ask for it, and consistency keeps your records aligned.
  • Treat the due date as the middle of a four-week window when planning leave and travel, not as a deadline.
  • If a first-trimester scan gives a different date, use the scan's date going forward; it is the more accurate anchor.

Frequently Asked Questions

How accurate is the due date?→
It is a statistical estimate: only about 4-5% of babies arrive on the exact date, and most are born within two weeks either side. A first-trimester ultrasound refines it.
Why does the calculator say I am 2 weeks further along than conception?→
Gestational age counts from the last period, roughly two weeks before conception. This is the universal medical convention.
What if my cycle is not 28 days?→
Naegele's rule assumes 28 days. With longer or shorter cycles the true date shifts; your midwife may adjust, and an early ultrasound settles it.
Which date does my doctor use?→
Care providers generally use the LMP-based estimate unless an early ultrasound differs meaningfully, in which case the scan date wins.

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