Pregnancy Week Calculator by LMP
Use the first day of your last menstrual period (LMP) to estimate your current pregnancy week, due date, and trimester.
Important: This is an informational estimate, not medical advice. Always confirm dates and growth milestones with your OB-GYN or midwife.
How this pregnancy week calculator works
A pregnancy week calculator by LMP estimates how far along a pregnancy is based on the first day of your last menstrual period. This is the standard method used in many clinics, especially early in pregnancy before ultrasound dating is available.
In obstetrics, pregnancy length is often counted as about 280 days (40 weeks) from LMP for a 28-day cycle. If your cycle is longer or shorter, this calculator adjusts the due date estimate.
What your results mean
1) Gestational age
This is the total time since your LMP. For example, if your result says 10 weeks and 3 days, that means 10 full weeks plus 3 days have passed since your LMP.
2) Pregnancy week
Pregnancy weeks are usually written as “Week 11,” “Week 22,” and so on. If you are 10 weeks and 3 days, you are in Week 11.
3) Trimester
- First trimester: Weeks 1 to 13
- Second trimester: Weeks 14 to 27
- Third trimester: Week 28 onward
4) Estimated due date
Your estimated due date (EDD) is the day your pregnancy reaches approximately 40 weeks. Most babies are born within a range around this date, not exactly on the EDD.
How to use this calculator correctly
- Enter the exact first day of your last menstrual period.
- Use your typical cycle length (28 days is common).
- Click Calculate to view your pregnancy week and due date estimate.
- Discuss the result with your healthcare provider, especially if your cycles are irregular.
Why LMP-based dating can differ from ultrasound
LMP-based dating assumes ovulation happened around the middle of your cycle. Real life can vary. If ovulation occurred earlier or later, dates can shift. That is why many providers confirm or revise estimated due dates after first-trimester ultrasound.
Quick accuracy notes
- Most accurate for people with regular menstrual cycles.
- Less accurate with irregular periods, recent hormonal contraception, or uncertain LMP date.
- Not a diagnostic tool for complications, viability, or fetal health.
When to contact your provider
Contact your doctor or midwife if you have severe pain, heavy bleeding, fever, persistent vomiting, fainting, or any symptom that concerns you. For emergencies, seek immediate medical care.
Frequently asked questions
Is this calculator the same as a due date calculator?
Yes, this tool includes due date estimation from LMP while also showing your current pregnancy week and trimester.
Can I use this with irregular cycles?
You can, but the estimate may be less precise. Ultrasound dating is often more reliable for irregular cycles.
Why does pregnancy count start before conception?
Medical dating starts from LMP because it provides a standardized reference point, even though conception usually happens about two weeks later in a 28-day cycle.
Bottom line
A pregnancy week calculator by LMP is a helpful first estimate for gestational age and expected due date. Use it as a planning tool, then confirm your timeline through prenatal care and professional medical assessment.