Pregnancy Week Calculator
Calculate pregnancy week quickly with our pregnancy tool. Get results based on evidence-based formulas with clear explanations.
Formula
Due Date = LMP + 280 days (Naegele's Rule)
The estimated due date is calculated by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP). Current gestational week = days since LMP / 7. Trimester: 1st = weeks 1-12, 2nd = weeks 13-26, 3rd = weeks 27-40. If due date is known, LMP is back-calculated as due date minus 280 days.
Worked Examples
Example 1: LMP Date Input — Early Pregnancy
Problem: Last menstrual period was January 15, 2026. What is the current pregnancy status? (Assume today is March 21, 2026.)
Solution: LMP: January 15, 2026\nDue date (LMP + 280 days): October 22, 2026\nDays since LMP: 65 days\nCurrent week: 65 / 7 = Week 9, Day 2\nTrimester: 1st (weeks 1-12)\nDays remaining: 280 - 65 = 215 days\nBaby size comparison: Cherry
Result: Week 9, Day 2 | 1st Trimester | Due: October 22, 2026 | 215 days remaining
Example 2: Due Date Input — Late Pregnancy
Problem: Due date is April 15, 2026. What is the current status? (Assume today is March 21, 2026.)
Solution: Due date: April 15, 2026\nLMP (due date - 280 days): July 9, 2025\nDays since LMP: 256 days\nCurrent week: 256 / 7 = Week 36, Day 4\nTrimester: 3rd (weeks 27-40)\nDays remaining: 280 - 256 = 24 days\nBaby size comparison: Romaine lettuce
Result: Week 36, Day 4 | 3rd Trimester | Due: April 15, 2026 | 24 days remaining
Frequently Asked Questions
How is pregnancy due date calculated?
Pregnancy due date is most commonly calculated using Naegele's rule, which adds 280 days (40 weeks) to the first day of the last menstrual period (LMP). This assumes a 28-day menstrual cycle with ovulation occurring on day 14. The formula was developed by German obstetrician Franz Naegele in the early 1800s and remains the standard method used by healthcare providers today. However, only about 4% of babies are born on their exact due date. Most babies arrive within two weeks before or after the estimated date. Ultrasound dating in the first trimester provides more accurate dating, especially for women with irregular cycles, and may adjust the estimated due date by several days.
What are the three trimesters of pregnancy?
Pregnancy is divided into three trimesters, each approximately 13 weeks long. The first trimester (weeks 1-12) is the period of rapid organ development when all major body systems form. This is when morning sickness is most common and the risk of miscarriage is highest. The second trimester (weeks 13-26) is often called the golden period because many uncomfortable symptoms subside, energy increases, and the baby bump becomes visible. Fetal movements (quickening) are typically first felt around week 20. The third trimester (weeks 27-40) involves rapid fetal growth and brain development. The baby gains most of its weight during this period, and the mother may experience increased fatigue, back pain, and Braxton Hicks contractions.
When should I first see a doctor during pregnancy?
Most healthcare providers recommend scheduling your first prenatal visit between weeks 6 and 8 of pregnancy, counted from the first day of your last menstrual period. At this visit, the provider confirms the pregnancy with blood work and potentially an early ultrasound, establishes the due date, reviews medical history, orders baseline lab work, and discusses prenatal vitamins. However, you should contact your provider immediately if you experience severe abdominal pain, heavy bleeding, high fever, or severe vomiting causing dehydration. After the initial visit, prenatal appointments are typically monthly until week 28, every two weeks until week 36, then weekly until delivery. More frequent monitoring may be recommended for high-risk pregnancies.
How accurate are the results from Pregnancy Week Calculator?
All calculations use established mathematical formulas and are performed with high-precision arithmetic. Results are accurate to the precision shown. For critical decisions in finance, medicine, or engineering, always verify results with a qualified professional.
How do I get the most accurate result?
Enter values as precisely as possible using the correct units for each field. Check that you have selected the right unit (e.g. kilograms vs pounds, meters vs feet) before calculating. Rounding inputs early can reduce output precision.
Does Pregnancy Week Calculator work offline?
Once the page is loaded, the calculation logic runs entirely in your browser. If you have already opened the page, most calculators will continue to work even if your internet connection is lost, since no server requests are needed for computation.