How the pregnancy due date calculator works
This calculator estimates your baby's due date from the first day of your last menstrual period (LMP), and shows your current gestational age and trimester. It's the same basic method used by most obstetricians for an initial estimate before an ultrasound provides a more individualized figure.
Formula and methodology
The calculation uses Naegele's rule, the standard obstetric formula: due date = LMP + 280 days (40 weeks), which assumes a typical 28-day menstrual cycle with ovulation around day 14. Gestational age (how far along the pregnancy is) is calculated as the number of days elapsed since the LMP, expressed in completed weeks and remaining days ("14w 3d," for example). Trimesters are divided as: 1st trimester from week 0 up to week 14 (day 0โ97), 2nd trimester from week 14 to week 28 (day 98โ195), and 3rd trimester from week 28 onward (day 196+) โ these boundaries vary slightly by source, but this is the most commonly used breakdown.
It's worth understanding the built-in limitation: Naegele's rule assumes a "textbook" 28-day cycle, so if your actual cycle is longer or shorter, or ovulation happened earlier or later than day 14, the estimate can be off by several days to a couple of weeks in either direction. That's exactly why an ultrasound-based dating scan, usually done in the first trimester, is considered more accurate and will typically supersede this calculation once you've had one.
Worked examples
Example 1: LMP was January 1, 2026. Due date = January 1 + 280 days = October 8, 2026.
Example 2: LMP was June 15, 2026; today is September 26, 2026. That's 103 days elapsed = 14 weeks, 5 days โ just into the 2nd trimester.
Example 3: For someone with a known longer cycle (say, 35 days instead of 28), their true ovulation and conception likely happened about a week later than Naegele's rule assumes, so their actual due date may run roughly a week later than this calculator's estimate โ a good example of why this is a starting estimate, not a diagnosis.
Frequently asked questions
How accurate is a due-date calculator?
It gives a reasonable estimate for someone with a standard 28-day cycle, but only about 4โ5% of babies are actually born exactly on their calculated due date โ most arrive within a 2-week window either side, which is considered entirely normal.
What if I don't know my exact LMP date?
If your cycle is irregular or you're unsure of the date, an early ultrasound dating scan (typically 8โ13 weeks) is significantly more reliable than this or any LMP-based calculation.
Is this the same method my doctor uses?
Naegele's rule is the standard first-pass estimate used in most clinical settings before a dating ultrasound, so yes, this mirrors the initial calculation โ but your provider's ongoing estimate should always take precedence as your pregnancy progresses.
Common mistakes people make
The most common mistake is confusing the LMP date with the estimated conception date โ they're roughly two weeks apart for a typical cycle, and using the wrong one shifts every downstream calculation by two weeks. Another is treating the due date as a firm deadline rather than a statistical estimate with a normal range of about two weeks either side. People with irregular cycles sometimes also apply this calculator without adjusting for their actual average cycle length, which reduces its accuracy further.
Real-world use cases
Expectant parents use a due-date calculator for early planning purposes โ estimating maternity/paternity leave timing, understanding which trimester and prenatal appointments are coming up, tracking fetal development milestones week by week, and general excitement and preparation before the first confirming ultrasound. This tool is informational only and is not a substitute for advice from a qualified healthcare provider โ always confirm dates and progress with your doctor or midwife.