Your pregnancy week by week
Our pregnancy calculator combines three essential tools in one: estimated due date, gestational weeks, and estimated baby size. You only need the date of your last menstrual period to get all the information.
How is the due date calculated?
Due Date = Last Menstrual Period + 280 days + Cycle Adjustment
Cycle Adjustment = (Your cycle length − 28) days
Naegele's rule is the standard method used by obstetricians and gynecologists worldwide. It was developed by Dr. Franz Naegele in the 19th century and remains the basis for estimating the estimated due date.
The three trimesters of pregnancy
First Trimester (Weeks 1-12)
The most critical period of development. All the baby's major organs form:
| Week | Size | Development |
|---|---|---|
| 4 | Poppy seed | Embryo implantation |
| 8 | Raspberry (~3g) | Heart beats, limbs form |
| 12 | Lime (~14g) | Organs formed, moves actively |
Second Trimester (Weeks 13-26)
The stage of rapid growth. The baby grows significantly:
| Week | Size | Development |
|---|---|---|
| 16 | Avocado (~100g) | Movement can be felt |
| 20 | Banana (~300g) | Anatomy scan, possible sex |
| 24 | Corn cob (~600g) | Fetal viability milestone |
Third Trimester (Weeks 27-40)
The final maturation phase:
| Week | Size | Development |
|---|---|---|
| 28 | Eggplant (~1kg) | Opens eyes, dreams |
| 32 | Zucchini (~1.7kg) | Lungs maturing |
| 36 | Melon (~2.6kg) | Positions for birth |
| 40 | Pumpkin (~3.5kg) | Ready to be born! |
Only 5% of babies are born exactly on their estimated due date. Most are born between weeks 37 and 42, which is considered completely normal.
How to read the result
The calculator estimates gestational age from your last menstrual period assuming a regular 28-day cycle. If your cycle is irregular, the first-trimester ultrasound provides the most accurate dating. Baby size references are medians: healthy babies vary widely at every week.
The size and weight references are educational medians from clinical standards. Your doctor uses ultrasound measurements to track your baby's growth individually.
Frequently asked questions
With Naegele's rule: last menstrual period + 280 days, plus an adjustment for cycles longer or shorter than 28 days. It is the standard method in obstetrics.
Only about 5% of babies are born on the exact date. The normal window is 37-42 weeks; the estimate is more accurate when confirmed by first-trimester ultrasound.
Averages by week: ~14g at week 12, ~300g at week 20, ~600g at week 24, ~1kg at week 28, ~2.6kg at week 36 and ~3.5kg at week 40. Healthy variation is wide.
First trimester: prenatal vitamins and early check-up. Second: anatomy scan (week 20) and glucose test. Third: birth plan, hospital bag, and more frequent check-ups.
Related calculators
- Due date: estimate your due date precisely
- Pregnancy weeks: current week and trimester
- Pregnancy weight gain: recommended ranges
- Ovulation: estimate conception dates