Pregnancy Calculator — Due Date Calculator
Calculate your due date, current gestational age, trimester, and key pregnancy milestones based on your last menstrual period (LMP). See also Age Calculator and BMI Calculator.
How to Calculate Your Due Date
The most common method for calculating a due date is Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day menstrual cycle with ovulation occurring on day 14. While only about 5% of babies are born on their exact due date, most are born within 1-2 weeks of it. The due date gives healthcare providers a reference point for monitoring fetal development and planning care.
Due Date Formula (Naegele's Rule)
Due Date = LMP + 280 days
Due Date = LMP + 40 weeks
Alternative: LMP − 3 months + 7 days + 1 year
Example
LMP: January 1, 2026
Due Date = January 1 + 280 days
Due Date = October 8, 2026
Conception (approx): January 15, 2026
Understanding Trimesters
Pregnancy is divided into three trimesters, each lasting approximately 13-14 weeks:
First Trimester (Weeks 1-12)
Major organ development, morning sickness common, highest risk of miscarriage. By week 12, all major organs are formed.
Second Trimester (Weeks 13-26)
Often called the "golden period" — morning sickness usually subsides, baby movements felt, anatomy scan at week 20.
Third Trimester (Weeks 27-40)
Rapid weight gain, baby's lungs mature, preparation for delivery. Full term begins at week 37.
Pregnancy Week-by-Week Size Chart
| Week | Size Comparison | Length | Weight |
|---|---|---|---|
| 8 | Raspberry | 1.6 cm | 1 g |
| 12 | Lime | 5.4 cm | 14 g |
| 16 | Avocado | 11.6 cm | 100 g |
| 20 | Banana | 16.4 cm | 300 g |
| 24 | Corn ear | 30 cm | 600 g |
| 28 | Eggplant | 37.6 cm | 1 kg |
| 32 | Squash | 42.4 cm | 1.7 kg |
| 36 | Honeydew melon | 47.4 cm | 2.6 kg |
| 40 | Watermelon | 51.2 cm | 3.4 kg |
Frequently Asked Questions
How accurate is the due date calculation?
Only about 5% of babies are born on their exact due date. Most are born within 2 weeks before or after. The due date is an estimate based on a 28-day cycle. Ultrasound dating in the first trimester is more accurate, especially for women with irregular cycles.
What if I don't know my last period date?
If you are unsure of your LMP, an early ultrasound (before 12 weeks) can estimate gestational age and due date with accuracy of plus or minus 5-7 days. Your healthcare provider can help determine the most accurate due date.
What is the difference between gestational age and fetal age?
Gestational age is counted from the first day of the last menstrual period (LMP), which is about 2 weeks before conception. Fetal age (also called embryonic age) is counted from the actual date of conception. Gestational age is always about 2 weeks more than fetal age.
When is a baby considered full term?
A baby is considered early term at 37-38 weeks, full term at 39-40 weeks, late term at 41 weeks, and post-term at 42+ weeks. The ideal delivery window is 39-40 weeks when the baby's brain, lungs, and liver are fully developed.
Solved Examples
Example 1: Calculate Due Date from LMP (Last Period: March 5, 2026)
Solution:
Step 1: Apply Naegele's Rule: LMP + 280 days = Due Date
Step 2: March 5 + 280 days = December 10, 2026
Step 3: Alternative method: March 5 − 3 months = December 5, then + 7 days = December 12
Step 4: Estimated conception date: March 5 + 14 days = March 19, 2026
Answer: Due date is approximately December 10, 2026
Example 2: Determine Current Trimester (LMP: January 15, 2026, Today: June 1, 2026)
Solution:
Step 1: Days since LMP: Jan 15 to Jun 1 = 16 (Jan) + 28 (Feb) + 31 (Mar) + 30 (Apr) + 31 (May) + 1 (Jun) = 137 days
Step 2: Current week: 137 ÷ 7 = 19 weeks, 4 days
Step 3: Trimester check: Week 19 falls in 13-26 range = Second Trimester
Step 4: Due date: January 15 + 280 = October 22, 2026 (143 days remaining)
Answer: Currently in Week 19, Day 4 — Second Trimester (due Oct 22)
Example 3: Calculate Gestational Age from Conception Date (Conceived April 10, 2026)
Solution:
Step 1: Estimated LMP = Conception date − 14 days = March 27, 2026
Step 2: Due date = March 27 + 280 days = January 1, 2027
Step 3: Gestational age (as of June 15, 2026) = 80 days from LMP = 11 weeks, 3 days
Step 4: Fetal age = Gestational age − 2 weeks = 9 weeks, 3 days
Answer: Gestational age = 11 weeks 3 days, Due date = January 1, 2027
Practice Questions
Q1: LMP was June 20, 2026. What is the estimated due date?
Answer: June 20 + 280 days = March 27, 2027. Alternative: June 20 − 3 months = March 20, + 7 days = March 27.
Q2: A woman is at week 28. Which trimester is she in and how many weeks remain?
Answer: Week 28 is the start of the Third Trimester (weeks 27-40). Weeks remaining: 40 − 28 = 12 weeks (84 days).
Q3: If conception occurred on February 1, 2026, what is the estimated LMP and due date?
Answer: LMP = Feb 1 − 14 days = January 18, 2026. Due date = Jan 18 + 280 = October 25, 2026.
Q4: A woman with LMP April 1 is told she's 50% through her pregnancy. What week is she in?
Answer: 50% of 280 days = 140 days = 20 weeks. She is at the halfway point (Week 20 anatomy scan).
Q5: A woman has a 35-day cycle (not 28). LMP: May 10, 2026. How does this affect her due date?
Answer: Standard Naegele: May 10 + 280 = Feb 14, 2027. With 35-day cycle adjustment (+7 days for late ovulation): Feb 21, 2027. Longer cycles push the due date later.
Q6: At what gestational week does the baby reach viability (can survive outside the womb)?
Answer: The viability milestone is generally considered to be Week 24, when lungs have developed enough surfactant for breathing. Survival rates at 24 weeks are approximately 40-70% with intensive NICU care.
Common Mistakes to Avoid
The most common mistake in pregnancy date calculation is confusing the Last Menstrual Period (LMP) date with the conception date — gestational age is counted from the LMP, which is approximately 2 weeks before conception actually occurred. Using the conception date as LMP will give a due date that is 2 weeks too early. Another frequent error is not accounting for irregular cycles: Naegele's Rule assumes a 28-day cycle with ovulation on day 14, but women with 35-day cycles ovulate around day 21, pushing the actual due date about 1 week later. People also place too much certainty on the due date — only 5% of babies arrive on the exact predicted date, and a normal delivery window spans 37-42 weeks. Confusing gestational age (from LMP) with fetal age (from conception) is another common mistake that creates a consistent 2-week discrepancy. Finally, early ultrasound dating (before 12 weeks) is more accurate than LMP-based calculations for women who don't remember their LMP or have irregular cycles.
Key Takeaways
- Due date = LMP + 280 days (40 weeks). Only about 5% of babies are born on their exact due date.
- Gestational age is counted from LMP (2 weeks before conception), while fetal age counts from actual conception.
- Pregnancy is divided into three trimesters: First (weeks 1-12), Second (weeks 13-26), Third (weeks 27-40).
- Irregular cycles affect accuracy — add the number of days your cycle exceeds 28 to your Naegele due date.
- Full term is 39-40 weeks. Early term (37-38) and late term (41 weeks) are acceptable but not ideal for delivery.
- First trimester ultrasound dating (before 12 weeks) is accurate to ±5-7 days and is more reliable than LMP for irregular cycles.