Pregnancy Due Date Calculator
Enter your last period or conception date to estimate your due date and current pregnancy week — all calculated in your browser.
Enter the dates to get the due date and gestational age.
※ An estimate based on Naegele's rule (LMP + 280 days, or conception + 266 days). Actual delivery dates vary widely — only about 4–5% arrive on the exact due date. Confirm dating with an ultrasound.
🔒 Your dates are calculated in the browser and never sent to a server.
How your due date is calculated (Naegele's rule)
The estimated due date (EDD) is most often found by <strong>adding 280 days — 40 weeks — to the first day of your last menstrual period (LMP)</strong>. This is Naegele's rule, named after the 19th-century German obstetrician Franz Naegele, and it remains the standard used by ACOG and clinics worldwide. By hand, you get the same date by taking the first day of the LMP, subtracting 3 months, then adding 7 days and one year. For example, an LMP of January 1, 2025 gives a due date of October 8, 2025. The rule assumes a regular 28-day cycle with ovulation on day 14. Real pregnancy length varies from person to person, but until an ultrasound confirms the dating, this LMP-based estimate is the default starting point. Just enter the date and the calculator applies the same rule instantly.
LMP basis vs. conception/ovulation basis
This tool supports two bases. The first adds <strong>280 days to your last menstrual period</strong>; the second adds <strong>266 days to a known conception or ovulation date</strong>. The 14-day gap between them is the average time from the start of your period to ovulation, which is why conception + 266 days lands on the same date as LMP + 280 days. If you know the exact date of conception — for example through IVF or ovulation testing — the conception basis may fit better. If your cycle is longer or shorter than 28 days and you don't know your ovulation date, the LMP basis is a safe default. Note that gestational age is always counted from the LMP by convention, while fetal (conceptional) age is about two weeks shorter.
Reading gestational age and trimesters
Gestational age is <strong>the number of days from your LMP to the reference date, divided into completed weeks plus leftover days</strong> — for instance, 159 days is 22 weeks and 5 days. Change the reference date to see exactly how far along you are on any given day. Pregnancy is grouped into three trimesters: the <strong>1st runs from 0 through 13 weeks 6 days, the 2nd from 14 through 27 weeks 6 days, and the 3rd from 28 weeks to birth</strong>. Each trimester brings different fetal development, changes for the mother, and recommended screenings, so knowing your current stage helps with planning. The due date falls at 40 weeks 0 days, and clinics generally treat birth between 37 and 42 weeks as full-term.
Your due date is an estimate
A due date is exactly that — an <strong>estimate</strong>. Only about 4-5% of babies arrive on the predicted day; most are born within the surrounding weeks, and spontaneous labor peaks around week 39. Birth anywhere from 37 to 42 weeks is considered a normal range, so treat the date as a target window rather than a fixed appointment. This calculator applies the standard Naegele's-rule formula as a <strong>planning aid — it is not a medical diagnosis and does not replace prenatal care.</strong> If your cycle is irregular or you're unsure of your LMP, an ultrasound at the clinic is the most accurate method. Every calculation runs entirely in your browser, and the dates you enter are never sent to or stored on any server.
Frequently asked questions
What if my cycle isn't 28 days?
Naegele's rule assumes a 28-day cycle with ovulation on day 14. A longer cycle can push the due date later and a shorter one earlier. If you know your ovulation or conception date, use the conception basis (+266 days) for a closer estimate, or confirm with an ultrasound.
Will my baby actually arrive on the due date?
Only about 4-5% of babies are born on their exact due date. Most arrive in the weeks around it, spontaneous labor is most common near week 39, and birth between 37 and 42 weeks is full-term. Treat the date as a target window.
Is the conception date more accurate?
If you know the precise date of conception (for example via IVF or ovulation testing), the conception basis can be more accurate. Keep in mind gestational age is conventionally counted from the LMP, so check the LMP-based result too if you want it to match clinic records.
What if an ultrasound gives a different date?
This calculator is a date-formula estimate only. Early-pregnancy ultrasounds measure the actual fetus and are more accurate, so if the results differ, the clinic's ultrasound-based due date should take precedence.