Pregnancy Due Date Calculator
Enter the first day of your last menstrual period to find your estimated due date, how far along you are, your trimester and when common US prenatal tests usually happen.
Enter the first day of your last period and tap Calculate due date.
Typical US prenatal tests and milestones
How your due date is calculated
Pregnancy is counted from the first day of your last menstrual period (LMP), not from conception. That is why you are already about 2 weeks “pregnant” when you conceive, and about 4 weeks when a home pregnancy test usually turns positive.
Naegele’s rule
The estimated due date is LMP + 280 days (40 weeks). The pencil-and-paper version is “add one year, subtract three months and add seven days”: for an LMP of March 1, 2026 it gives December 8, 2026. Adding exactly 280 days gives December 6, 2026: the shortcut can be a day or two off because months have different lengths, so this calculator always adds exactly 280 days.
Adjusting for cycle length
Naegele’s rule assumes a 28-day cycle with ovulation on day 14. If your cycles are longer or shorter, ovulation usually moves by the same amount, so the calculator adds (cycle length − 28) days. With a 32-day cycle the due date moves 4 days later. The week count is adjusted the same way, so your due date is always exactly 40 weeks 0 days.
Early ultrasound comes first
ACOG recommends that the due date be confirmed by ultrasound in the first trimester, when measuring the baby’s length (crown–rump length) is the most accurate way to date a pregnancy. If your doctor or midwife gives you a different due date after an ultrasound, use theirs.
Typical prenatal test timeline in the US
| When | Test or milestone | What it is |
|---|---|---|
| From 10 weeks | Cell-free DNA screening (NIPT) | Optional blood test that screens for Down syndrome and some other chromosome conditions |
| 10–13 weeks | First-trimester screening | Blood test plus nuchal translucency (NT) ultrasound, usually done together |
| 15–22 weeks | Quad screen | Second-trimester blood test for Down syndrome, trisomy 18 and neural tube defects |
| 18–22 weeks | Anatomy ultrasound | The standard mid-pregnancy scan that checks the baby’s organs and growth |
| 24–28 weeks | Glucose screening | Checks for gestational diabetes (earlier if you have risk factors) |
| 27–36 weeks | Tdap vaccine | Recommended in every pregnancy, as early as possible in this window, to protect the newborn from whooping cough |
| 36–37 weeks | Group B strep (GBS) test | Vaginal–rectal swab, done between 36 and 38 weeks |
| 39–40 weeks | Full term | ACOG: early term 37–38 weeks, full term 39 0/7–40 6/7, late term 41 weeks, post-term 42 weeks and beyond |
Screening tests are offered, not required, and the exact timing depends on your health and your care provider. Follow the schedule your doctor or midwife gives you.
Trimesters and frequently asked questions
| Trimester | Weeks | What’s happening |
|---|---|---|
| First | 0 – 13w 6d | Major organs form; nausea and tiredness are common |
| Second | 14w – 27w 6d | Many people feel the first movements; often the most comfortable stretch |
| Third | 28w – birth | The baby gains weight quickly and your body prepares for labor |
Will my baby arrive on the due date?
Probably not exactly. Only a small share of babies, often quoted as around 5%, are born on their due date. Most arrive within a couple of weeks either side of it.
My ultrasound date is different. Which is right?
If your cycles are irregular or you ovulated late, an early ultrasound is more accurate than the LMP. Your provider may change your due date based on it, and that date is the one to use.
What if I don’t remember my last period?
Enter your best estimate, or ask your provider about a dating ultrasound. Planning for conception? Try the Ovulation Calculator.
Sources
- ACOG Committee Opinion No. 700, Methods for Estimating the Due Date.
- ACOG Committee Opinion No. 579, Definition of Term Pregnancy.
- ACOG FAQ, Prenatal Genetic Screening Tests (cell-free DNA, first-trimester screening, quad screen).
- ACOG FAQ, Ultrasound Exams (standard exam at 18–22 weeks).
- ACOG FAQ, Routine Tests During Pregnancy (glucose screening, GBS).
- ACOG Committee Opinion No. 718, Update on Immunization and Pregnancy: Tetanus, Diphtheria, and Pertussis Vaccination.