Overview
This page estimates a due date and current gestational age from the first day of the last menstrual period using Naegele's rule: LMP plus 280 days, adjusted for your actual cycle length. It also shows the current trimester and a days-remaining figure. The calculation is instant, runs entirely in your browser, and nothing you type is sent anywhere.
How It Works
Enter the first day of your last menstrual period in the date field, then set your typical cycle length in days (the input accepts 20 to 45 and defaults to 28). The page immediately shows the estimated due date, today's gestational age in weeks and days, which trimester you are in, and an estimated number of days to go. The date field starts filled with today, so change it before reading the numbers. The result recalculates the moment either input changes, and the page keeps working offline once loaded.
Step-by-Step Usage Guide
- Enter the first day of your last menstrual period.
- Set your usual cycle length if it is not close to 28 days.
- Read the estimated due date and today's gestational age in the result cards.
- Re-check the page as weeks pass; the gestational age updates against today's date.
Technical Specifications & Standards
The rule implemented is the classic Naegele estimate: 280 days (40 weeks) from the first day of the last menstrual period, which assumes ovulation on day 14 of a 28-day cycle. The code adjusts for cycle length by adding (cycle - 28) days to the 280-day base, so a 32-day cycle pushes the estimate back by 4 days. Gestational age is counted from the LMP, not from conception - by this convention day 7 of a pregnancy is week one complete even though conception happened roughly two weeks in. Trimester boundaries are drawn at 91 and 189 gestational days. One engineering detail: the due date is formatted from local calendar date parts rather than the toISOString method, which would shift the displayed calendar day for anyone east or west of UTC. Validation research shows Naegele's rule is only an average; ovulation timing varies, and first-trimester ultrasound dating is more accurate than any LMP calculation.
Targeted Use Cases
- Getting a preliminary due date in the days before a first prenatal appointment.
- Working out which trimester a given week falls into for planning work or travel.
- Checking a rough gestational age when cycles are regular and well tracked.
- Estimating a date for a pregnancy announcement before the first scan.
Notes & Gotchas
- Use the cycle-length field whenever your cycle is reliably longer or shorter than 28 days.
- Enter dates from your records rather than guessing the LMP.
- Treat the result as a window of a week or more, not a single promised day.
- Bring your LMP date to the first ultrasound appointment for comparison.
Frequently Asked Questions
How does the calculator adjust for irregular cycles?
It shifts the 280-day base by the difference between your cycle length and 28 days, because later ovulation means a later due date. Cycles between 20 and 45 days are accepted.
Can a due date from LMP be wrong?
Yes. Naegele's rule assumes a 28-day cycle with ovulation on day 14. Real ovulation timing varies, which is why ultrasound measurements, especially in the first trimester, are the medical standard for dating a pregnancy.
Is this a medical tool?
No. It is an educational estimate of the kind given by standard Naegele's-rule charts. Only a clinician with ultrasound data can give you a medically usable due date.
Why does the gestational age count from my period rather than conception?
Gestational age is an international convention counted from the LMP, about two weeks before conception. It exists so clinicians everywhere use the same clock, not because it reflects embryonic age.