Overview
What this calculator does
Estimate your pregnancy due date from your last menstrual period (LMP), your conception date, or an IVF transfer date. The calculator also shows your estimated conception date, the boundaries of each trimester, and key pregnancy milestones. It is an estimate for planning — your healthcare provider's dating is the most accurate.
The math
How the due date is calculated
The standard method is Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period.
- From LMP: due date = LMP + 280 days. Cycle length adjusts the estimate if it isn't 28 days.
- From conception: conception happens about 14 days after LMP, so due date = conception + 266 days.
- From IVF transfer: the timing is known precisely. A day-5 (blastocyst) transfer gives due date = transfer + 261 days.
The 40-week timeline
How pregnancy weeks are counted
You're '2 weeks pregnant' at conception
What to expect when
The trimesters and milestones
| Weeks | What it marks | |
|---|---|---|
| First trimester | Weeks 1-13 | Organs form; highest miscarriage risk early |
| 12 weeks | End of T1 | Risk drops; often when news is shared |
| Second trimester | Weeks 14-27 | The 'easier' middle stretch |
| 20 weeks | Halfway | Anatomy scan; often find out the sex |
| 24 weeks | Viability | Survival outside the womb becomes possible |
| Third trimester | Weeks 28-40 | Rapid growth; prepare for birth |
| 37 weeks | Full term | Baby is considered full term from here |
Set expectations
How accurate is a due date?
A due date is an estimate, not a deadline. Only about 1 in 20 babies (5%) are born on their exact due date. Most arrive within two weeks on either side. A "normal" delivery window is 37 to 42 weeks.
The most accurate dating comes from an early ultrasound (before 13 weeks), which your provider uses to confirm or adjust the date. LMP-based estimates are less reliable if your cycles are irregular or you're unsure of your last period.
Behind the scenes
Privacy and how it runs
Your dates stay private
Common questions
How many weeks pregnant am I?
Count from the first day of your last period to today. If that was 10 weeks ago, you're about 10 weeks pregnant. The calculator works from that same starting point to estimate your due date.
Can the due date change?
Yes. Providers often adjust it after an early ultrasound, which measures the baby directly and is more accurate than LMP, especially for irregular cycles. A later change is uncommon unless early dating was uncertain.
What if I don't know my last period date?
Use the conception date if you know it, or rely on an ultrasound from your provider. LMP-based dating needs that first-day date to be reasonably accurate.
Does cycle length really matter?
Yes, somewhat. Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycle is regularly longer or shorter, ovulation shifts and so does the due date. Enter your average cycle length for a closer estimate.
Is this a medical tool?
No. It's an estimate for planning and curiosity. Always get prenatal care and confirm your due date with a qualified healthcare provider.
Related calculators
Date and health tools that pair with the due date calculator.
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