Pregnancy Due Date Calculator
Use this pregnancy calculator to estimate your due date, current week of pregnancy, and conception date — whether you know your last period date, an estimated conception date, or an existing due date from a doctor. Get your trimester breakdown and full pregnancy timeline instantly.
How Does a Pregnancy Due Date Calculator Work?
This pregnancy calculator estimates your due date using one of three starting points: the first day of your last menstrual period, an estimated conception date, or an existing due date you already have from a doctor. Whichever method you use, the calculator also works out your estimated conception date, current week of pregnancy, and full trimester timeline.
The most common calculation method is Naegele’s rule, described by ACOG: add 280 days (40 weeks) to the first day of your last period. This assumes a fairly standard 28-day cycle with ovulation around day 14 — this calculator adjusts slightly for your actual average cycle length if you provide it.
Due Date vs. Conception Date vs. Last Period Date
These three dates are related but distinct, and mixing them up is a common source of confusion:
- Last period date — the first day of your most recent period. Pregnancy weeks are counted from this date, even though conception happens later.
- Conception date — when fertilization actually occurred, typically around ovulation, about two weeks after your last period began.
- Due date — the estimated date of birth, calculated as 280 days from your last period (or 266 days from conception).
This is why a pregnancy conception calculator and a due date calculator often ask for different starting information, but ultimately describe the same pregnancy timeline from different reference points.
How Accurate Is a Due Date Calculator?
Important: A due date is an estimate, not a guarantee. Only about 5% of babies are actually born on their exact estimated due date.
Most full-term babies are born somewhere between two weeks before and two weeks after the estimated due date. Calculator-based estimates using your last period assume a standard cycle length and ovulation timing, which don’t apply equally to everyone — particularly if your cycles are irregular. An early ultrasound, typically done in the first trimester, is often considered more precise for dating a pregnancy, since it directly measures fetal size rather than relying on cycle assumptions.
Understanding Your Trimesters
| Trimester | Weeks | General Focus |
|---|---|---|
| First | Weeks 1–13 | Early development; many people experience early pregnancy symptoms |
| Second | Weeks 14–27 | Often considered the most comfortable stretch for many people |
| Third | Weeks 28–40 | Final growth and preparation for birth |
These week ranges are general guidelines, not strict boundaries — individual pregnancies vary, and a healthcare provider will track your specific progress at each appointment.
How Many Weeks Pregnant Am I?
Pregnancy is measured in weeks starting from the first day of your last period — not from your conception date, even though conception happens about two weeks later. This can feel counterintuitive, since it means you’re already considered a couple of weeks “pregnant” before conception has actually occurred, but it’s the standard method used by healthcare providers everywhere because the start of your last period is a clear, known date, whereas the exact moment of conception usually isn’t. This pregnancy due date calculator handles that counting for you automatically, whichever starting point you use.
Comparing Due Date Calculation Methods
| Method | What It Needs | Best For |
|---|---|---|
| Last period date | First day of your last period, average cycle length | Most common starting point; works well with regular cycles |
| Conception date | Known or estimated date of conception | If you’re tracking ovulation closely or conceived via a known method |
| Known due date | A due date already given by a doctor or ultrasound | Working backward to estimate conception date and pregnancy timeline |
| Ultrasound dating | Fetal measurements from an early scan | Most precise, especially with irregular cycles — done by a healthcare provider |
If your calculator-based estimate and your doctor’s ultrasound-based estimate differ by a few days, the ultrasound date is generally considered more reliable, since it’s based on direct measurement rather than cycle assumptions. Whichever method you use, this pregnancy due date calculator supports all three input types on the same page.