Due Date Calculator (LMP)

About the Due Date Calculator (LMP)

Knowing your estimated due date (EDD) is one of the first and most exciting milestones of pregnancy. The LMP-based due date calculator uses Naegele's rule, the same method obstetricians have relied on for over a century, to estimate when your baby will arrive. Simply enter the first day of your last menstrual period and your average cycle length, and the calculator does the rest.

Naegele's rule assumes a standard 28-day cycle with ovulation occurring on day 14. The formula adds 280 days (40 weeks) to the first day of your LMP to arrive at the EDD. If your cycle is longer or shorter than 28 days, the calculator adjusts automatically: a 32-day cycle, for example, shifts the due date four days later because ovulation likely occurred four days later than the standard assumption.

Keep in mind that only about 5% of babies arrive exactly on their due date. Most births occur within a two-week window around the EDD. Your healthcare provider may adjust the date after an early ultrasound, but this calculator gives you an excellent starting point.

When This Page Helps

An accurate due date helps you and your care team plan prenatal visits, schedule screenings, and prepare for delivery. It also helps you understand which trimester you are in, track fetal development milestones, and make practical decisions like when to start maternity leave. Using the LMP method is the quickest way to get an initial EDD before your first ultrasound appointment.

How to Use the Inputs

1. Enter the first day of your last menstrual period (LMP) using the date picker.
2. Enter your average menstrual cycle length in days. The default is 28 days.
3. Click or tap to calculate. Your estimated due date, conception window, and key trimester dates will appear quickly.
4. Review the trimester breakdown to understand your pregnancy timeline.
5. Share the result with your healthcare provider at your next prenatal visit for confirmation.

Example Calculation

Tips & Best Practices

• Record the first day of bleeding, not spotting — that is your true LMP.
• If your cycles vary, average the last 3-6 cycles for a more accurate result.
• A cycle shorter than 28 days moves the due date earlier; longer moves it later.
• Your provider may adjust the EDD after a first-trimester ultrasound if measurements differ by more than 7 days.
• Only about 5% of babies are born on the exact due date — expect a 2-week window.
• Track your cycle with an app for several months before trying to conceive for the most accurate LMP data.

How Naegele's Rule Works

Naegele's rule is elegant in its simplicity: add 7 days to your LMP, subtract 3 months, and add 1 year. Mathematically, this equals adding 280 days. The formula assumes ovulation on cycle day 14, which is why adjusting for actual cycle length improves accuracy.

When to See Your Provider

Most providers schedule the first prenatal visit between weeks 8 and 12. At this appointment, an ultrasound may confirm or adjust your EDD. Between weeks 11 and 14, a nuchal translucency scan screens for chromosomal conditions.

Planning Around Your Due Date

Once you have an EDD, you can count backward to plan important milestones: when to announce the pregnancy, when to begin nursery preparation, when to take childbirth classes, and when to finalize your birth plan. Many parents also use the date to plan maternity and paternity leave.

Frequently Asked Questions

• What is Naegele's rule?

  Naegele's rule is a standard method for estimating due dates. It adds 280 days (40 weeks) to the first day of your last menstrual period, assuming a 28-day cycle. It was developed by German obstetrician Franz Naegele in the early 1800s and remains the most widely used formula.

• Why does cycle length matter?

  Cycle length affects when you ovulate. In a 28-day cycle, ovulation typically occurs on day 14. In a 35-day cycle, ovulation occurs around day 21 — a full week later. The calculator adjusts by adding or subtracting the difference from 28 days.

• How accurate is the LMP method?

  The LMP method is accurate to within about 2 weeks for most women with regular cycles. First-trimester ultrasound dating is slightly more precise. If both methods agree within 7 days, providers typically keep the LMP-based date.

• What if I don't remember my LMP?

  If you cannot recall your LMP, your provider will use an early ultrasound to date the pregnancy. You can also try our due-date-ultrasound or due-date-conception calculator as alternatives.

• Can I use this calculator if I have irregular periods?

  You can, but accuracy may be reduced. Enter the best estimate of your last period and average cycle length. An early ultrasound will provide a more reliable date for women with irregular cycles.

• Is the due date the day my baby will definitely be born?

  No. The due date is an estimate. Full-term delivery ranges from 37 to 42 weeks. About 80% of babies are born within 10 days of the EDD, and only 5% arrive on the exact date.

• What is the difference between gestational age and fetal age?

  Gestational age is counted from the first day of your LMP and is about 2 weeks longer than fetal age (which counts from conception). When a doctor says you are 8 weeks pregnant, that is gestational age — the embryo is approximately 6 weeks old.

• Should I change my due date after an ultrasound?

  If a first-trimester ultrasound differs from the LMP date by more than 7 days, most providers will adjust the EDD to the ultrasound date. After the first trimester, changes are less common unless the discrepancy is large.