Due Date Calculator

About the Due Date Calculator

A due date calculator estimates when a baby is likely to be born, giving expectant parents the single most anticipated date of a pregnancy and clinicians a reference point for scheduling prenatal care, tests, and screenings. The estimated due date (EDD) is calculated from the first day of the last menstrual period (LMP) using a method that has been standard in obstetrics for over a century. Although only about 4% of babies actually arrive on their exact due date, the estimate is far more than a curiosity: it anchors the entire pregnancy timeline, determining when key milestones like the first-trimester screening, the anatomy scan around 20 weeks, glucose testing, and the assessment of a full-term versus preterm or post-term birth all take place. Our calculator uses Naegele's rule, the traditional LMP-based method, to estimate your due date and current gestational age, and explains how factors such as an irregular cycle, a known conception date, or an early ultrasound can refine that estimate. It is an educational tool and not a substitute for the dating your healthcare provider will confirm.

How It Works

The standard method, Naegele's rule, estimates the due date by taking the first day of your last menstrual period, adding one year, subtracting three months, and adding seven days — which works out to roughly 280 days, or 40 weeks, from the start of your last period. This convention counts pregnancy from the LMP rather than from conception because the LMP is a date most people can identify, whereas the exact moment of conception usually cannot be pinpointed. It assumes a regular 28-day cycle with ovulation on day 14; if your cycle is consistently longer or shorter, the estimate shifts accordingly, because ovulation and therefore conception occurred later or earlier than day 14. Gestational age is expressed in completed weeks and days from the LMP, which is why you are considered "two weeks pregnant" before conception has even occurred under this dating system. Early ultrasound measurements, particularly the crown-rump length in the first trimester, provide the most accurate dating and often adjust the LMP-based estimate by several days. Our calculator applies Naegele's rule and can account for cycle length, but your provider's ultrasound-confirmed date takes precedence.

Tips & Best Practices

• ✓Only about 4% of babies are born on their exact due date. A full-term birth is anywhere from 37 to 42 weeks, so treat the due date as the centre of a window, not a deadline.
• ✓The due date is calculated from your last menstrual period, not conception, which is why you're counted as roughly two weeks pregnant before conception actually occurs under standard obstetric dating.
• ✓An early first-trimester ultrasound gives the most accurate dating. If it differs from your LMP-based estimate by more than about a week, your provider will usually adopt the ultrasound date.
• ✓If your cycles are irregular or longer than 28 days, the standard LMP calculation may be off, because ovulation and conception happened later than the assumed day 14. Mention this to your provider.
• ✓Knowing an accurate due date matters medically: it determines whether a baby is preterm, full-term, or post-term, and it times critical screenings like the 20-week anatomy scan and glucose testing.
• ✓First pregnancies tend to run slightly longer on average, though there is wide natural variation and no reliable way to predict your exact delivery day in advance.
• ✓This calculator is an educational estimate. Always confirm your due date and pregnancy dating with a qualified healthcare provider, who will use ultrasound and clinical assessment.
• ✓If you know your exact conception or IVF transfer date, that can produce a more precise estimate than LMP dating — share it with your provider so your timeline is calculated accordingly.

Who Uses This Calculator

Expectant parents use a due date calculator the moment they learn they are pregnant, to know roughly when their baby will arrive and to begin planning parental leave, childcare, and preparations. Clinicians and midwives use LMP-based dating as the starting reference for scheduling the full sequence of prenatal appointments and screening tests, each of which is timed to a specific gestational week. Parents tracking pregnancy week by week use the calculated gestational age to follow fetal development milestones and to understand which trimester they are in. People trying to conceive use it to project a potential due date from a planned or suspected conception. Employers and HR departments reference the estimated due date to plan for maternity and paternity leave coverage. Those with irregular cycles use the calculator to understand why their estimate may differ from a simple LMP calculation and why an early ultrasound will likely refine it. Couples use the due date to count down and to time announcements, baby showers, and hospital arrangements. In every case the estimate is a planning tool, and the actual delivery may occur any time within the normal full-term window.