Pregnancy Due Date Calculator
Calculate your estimated due date, current pregnancy week, trimester, and key milestones.
What is a Pregnancy Due Date Calculator?
A pregnancy due date calculator estimates when your baby is most likely to arrive, using the first day of your last menstrual period (LMP) or a confirmed conception date. The standard method, Naegele's rule, was proposed in the early 19th century by German obstetrician Franz Karl Naegele and remains the basis of modern pregnancy dating: a typical gestation lasts about 280 days (40 weeks) measured from the first day of the LMP, or 266 days (38 weeks) from conception. Although ultrasound dating is now more precise, LMP-based calculation remains the widely used first-line estimate in pregnancy-tracking apps, clinical forms, and public-health resources.
It is important to understand that a due date is an estimate, not a prediction. Only about 4–5% of babies are born on their exact due date; roughly 70–80% arrive within a two-week window around it. Factors that influence actual delivery date include maternal age, parity (first vs. subsequent pregnancy), genetics, ethnicity, and cycle regularity. For women with cycles longer or shorter than 28 days, a simple adjustment (adding or subtracting the difference) produces a more realistic estimate, which is why modern calculators ask for average cycle length.
Tracking the pregnancy week by week helps expectant parents prepare for each developmental milestone, plan prenatal appointments, and recognize when certain symptoms or screenings are expected. It also gives clinicians a shared timeline for ordering tests at the right gestational age — first-trimester combined screening around weeks 11–13, the anatomy scan at 18–22, and the glucose challenge at 24–28.
How is it Calculated?
Naegele's rule:Due date = first day of LMP + 280 days (or + 1 year − 3 months + 7 days).
Worked example: If the LMP started on January 1, 2026, the estimated due date is approximately October 8, 2026 (January + 9 months + 7 days). For a 32-day cycle, add four days to the result; for a 25-day cycle, subtract three days.
The Three Trimesters
- First trimester (weeks 1–12): rapid organ formation, common symptoms include nausea, fatigue, and breast tenderness.
- Second trimester (weeks 13–27): often the most comfortable phase; baby movements are felt, anatomy scan around week 20.
- Third trimester (weeks 28–40): rapid weight gain for the baby, Braxton Hicks contractions, preparation for birth.
Important Reminders
- This tool is for general information only; always consult a qualified obstetrician or midwife.
- First-trimester ultrasound is more accurate than LMP for dating, especially with irregular cycles.
- Take prenatal vitamins with folic acid (400–800 mcg/day) ideally before conception.
- Avoid alcohol, smoking, raw fish, and unpasteurized products; review medications with a doctor.
- Attend scheduled prenatal visits — early detection of complications improves outcomes dramatically.
Frequently Asked Questions
How accurate is a due date calculator?
Only 4–5% of babies arrive on the exact due date; most arrive within two weeks of it. First-trimester ultrasound is the most accurate dating method.
What if my cycle is not 28 days?
Adjust by the cycle-length difference — longer cycles push the due date later; shorter cycles bring it earlier.
When is the first ultrasound?
Typically between weeks 6 and 9 to confirm pregnancy; anatomy scan follows at 18–22 weeks.
What are the three trimesters?
Weeks 1–12 (first), 13–27 (second), and 28–40 (third).
Should I rely on this for medical decisions?
No. Always consult your obstetrician or midwife for accurate dating and medical advice.