Estimate Your Due Date, Gestational Age, and Trimester.
One of the first questions after a positive pregnancy test is almost always “when am I due?” Our free Pregnancy Calculator gives you an instant estimate — your due date, how far along you currently are, and which trimester you’re in — based on simple, widely used obstetric calculations.
How Due Dates Are Actually Calculated
The most common method, known as Naegele’s Rule, estimates a due date as 280 days (40 weeks) from the first day of your last menstrual period (LMP) — not from the date of conception, which is a common point of confusion. This might seem like an odd starting point since conception obviously happens later, but it’s used because the exact date of conception is rarely known with certainty, while the start of the last period usually is.
This method assumes a standard 28-day cycle with ovulation occurring around day 14. For those with longer or shorter cycles, actual conception — and therefore the most accurate due date — can shift earlier or later than this standard calculation suggests, which is one reason an early pregnancy ultrasound is often used by healthcare providers to confirm or adjust the estimated date based on the embryo’s measured size.
What Gestational Age and Trimester Actually Mean
Gestational age is how pregnancy progress is measured, and it’s counted from the first day of your last menstrual period, not from conception — meaning that in the very early weeks, you’re technically counted as a few weeks pregnant before conception has even occurred. This can feel counterintuitive, but it’s the standard method used throughout medical care, so it’s worth understanding rather than being confused by it later.
The three trimesters break pregnancy into roughly equal stages:
- First trimester — weeks 1 through 12, covering the earliest and most developmentally critical period
- Second trimester — weeks 13 through 26, often described as the most comfortable stage for many people
- Third trimester — weeks 27 through birth, as the body prepares for delivery
Knowing which trimester you’re in helps frame what to generally expect in terms of common symptoms, recommended prenatal appointments, and developmental milestones — though every pregnancy is different, and your own healthcare provider is the best source for what’s specifically happening in your case.
How to Use the Pregnancy Calculator
- Enter the first day of your last menstrual period.
- Get your estimated due date, current gestational age, and trimester, calculated instantly.
- Revisit as needed throughout your pregnancy to track how far along you are.
An Important Note on Accuracy
It’s worth being upfront about this: only a small percentage of babies are actually born on their calculated due date. The estimate from this calculator — and from Naegele’s Rule generally — gives you a useful general target, typically accurate within a window of a couple of weeks either direction, rather than a precise prediction. Actual due dates are refined over the course of pregnancy through ultrasound measurements and your healthcare provider’s ongoing assessment, which account for factors a calendar-based calculation simply can’t, including your specific cycle length and the baby’s actual growth.
This tool is meant to give you a helpful starting estimate and a general sense of where you are in your pregnancy — it isn’t a substitute for prenatal care. Regular appointments with an OB-GYN or midwife are essential throughout pregnancy, both for accurately confirming your due date and, far more importantly, for monitoring your health and your baby’s development throughout.
A Helpful Starting Point
Whether you just found out you’re expecting or want a quick way to check how far along you are, the Pregnancy Calculator gives you an instant, easy-to-understand estimate — a helpful starting point for a journey that your healthcare provider will guide you through in far more detail.
