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.