In-depth
How the Pregnancy Calculator Works
The pregnancy calculator allows you to find out what week you are in, the estimated due date, and the main milestones of your baby's development. Simply enter the date of your last menstrual period.
How Pregnancy Weeks Are Counted
Pregnancy weeks are counted from the first day of the last menstrual period (LMP - Last Menstrual Period), not from the day of conception. This means that during the first 2 "pregnancy" weeks, conception has not yet occurred.
The standard duration of pregnancy is 40 weeks (280 days) from the first day of the last menstrual period.
Naegele's Rule
The estimated due date is calculated using Naegele's Rule:
- Take the date of the first day of the last menstrual period
- Add 7 days
- Subtract 3 months
- Add 1 year
Example: Last menstrual period on January 1, 2026 → Estimated due date: October 8, 2026.
Only 5% of babies are born exactly on the predicted date. A delivery between the 37th and 42nd week is considered full-term.
The Three Trimesters
| Trimester | Weeks | Main Development |
|---|---|---|
| First | 1-12 | Organ formation, heartbeat, first movements |
| Second | 13-26 | Rapid growth, perceptible movements, morphology ultrasound |
| Third | 27-40 | Lung maturation, positioning, preparation for birth |
Key Check-ups and Tests
- Week 6-8: First ultrasound (pregnancy and heartbeat confirmation)
- Week 11-13: Nuchal translucency and combined screening test
- Week 15-18: Amniocentesis (if indicated)
- Week 19-21: Morphology ultrasound
- Week 24-28: Glucose tolerance test (gestational diabetes screening)
- Week 30-32: Growth ultrasound
- Week 35-37: Vaginal swab (Group B Streptococcus)
Months and Weeks: the Conversion
The conversion between months and weeks is not always intuitive:
- 1st month: weeks 1-4
- 2nd month: weeks 5-8
- 3rd month: weeks 9-13
- 4th month: weeks 14-17
- 5th month: weeks 18-22
- 6th month: weeks 23-27
- 7th month: weeks 28-31
- 8th month: weeks 32-35
- 9th month: weeks 36-40
