What is a Pregnancy Calculator?
A pregnancy calculator (also called a pregnancy estimator or due date calculator) estimates your expected delivery date based on the first day of your last menstrual period (LMP) or your conception date. It uses Naegele's rule, the standard medical formula endorsed by the NHS and WHO, to calculate a 40-week pregnancy timeline.
This free pregnancy calculator also tracks your pregnancy week by week, identifies your current trimester, and highlights key prenatal milestones — helping you plan appointments, scans, and preparations with confidence.
This tool uses Naegele's rule, the globally accepted formula for estimating due dates in clinical practice:
Understanding Pregnancy Trimesters
Pregnancy is divided into three trimesters, each with distinct developmental milestones:
| Trimester | Weeks | Key Developments |
|---|
| First | 1–13 | Embryo forms; heartbeat detectable ~week 6; major organs begin developing |
| Second | 14–27 | Rapid growth; movement felt ~week 18–22; anatomy scan ~week 20 |
| Third | 28–40+ | Lungs mature; baby gains weight; preparations for birth |
How to Use This Pregnancy Calculator
1
Choose Your Input Method
Select LMP Date if you know the first day of your last period (most common). Choose Conception Date if you tracked ovulation or had fertility treatment — this method is more precise when conception timing is known.
2
Enter Your Date & Cycle Length
Input your LMP or conception date using the date picker. If your cycle isn't 28 days, adjust the cycle length field. This fine-tunes your due date for greater accuracy with irregular periods.
3
Get Your Results Instantly
Your estimated due date appears instantly, along with your current pregnancy week, trimester, days remaining, and key milestones. Use this to plan prenatal appointments and preparations.
4
Track Week by Week
Bookmark this page or save your result. Return weekly to see updated progress, upcoming milestones, and trimester transitions. Share results with your partner or healthcare provider.
Accuracy & Medical Guidance
Naegele's rule is ~95% accurate for regular 28-day cycles. Accuracy may vary with:
- Irregular cycles: Adjust cycle length in the calculator or confirm with ultrasound.
- Uncertain LMP: If you're unsure of your last period date, use an early ultrasound (before 14 weeks) for the most accurate dating.
- Fertility treatments: If you had IVF or tracked ovulation precisely, use the conception date method for best results.
Important: Only ~5% of babies are born exactly on their due date. Most arrive between 37–42 weeks. Always confirm your due date with your NHS midwife or GP, who may adjust it based on ultrasound measurements.