Pregnancy Week by Week Calculator (LMP Method)
Use your last menstrual period (LMP) to estimate your current pregnancy week, trimester, and due date.
This tool provides estimates only and does not replace professional prenatal care.
What is an LMP pregnancy calculator?
A pregnancy week by week calculator by LMP estimates gestational age based on the first day of your last period. This is the same starting point used in most clinical settings before ultrasound dating is available. The calculator gives you a practical timeline so you can understand where you are in pregnancy right now and what comes next.
How this calculator works
1) Gestational age
Pregnancy length is measured from LMP, not conception date. In a typical 28-day cycle, this means pregnancy counting starts about two weeks before ovulation and fertilization.
2) Due date estimate
The common estimate adds 280 days (40 weeks) to your LMP. If your cycle is longer or shorter than 28 days, the tool adjusts your due date estimate accordingly.
3) Week-by-week position
The calculator displays your current pregnancy week and day (for example: Week 19, Day 3), your trimester, and a short milestone summary for that stage.
Pregnancy timeline overview by trimester
First trimester (Weeks 1–13)
- Weeks 1–4: Early hormonal changes begin; implantation typically occurs in this period.
- Weeks 5–8: Major early organ systems begin forming.
- Weeks 9–13: Growth continues rapidly; many early symptoms may change in intensity.
Second trimester (Weeks 14–27)
- Weeks 14–18: Energy may improve for many people.
- Weeks 19–22: Anatomy ultrasound is often scheduled around this window.
- Weeks 23–27: Continued growth, increasing fetal movement, and routine follow-up visits.
Third trimester (Weeks 28–40+)
- Weeks 28–32: Baby gains significant weight; care visits become increasingly focused on birth planning.
- Weeks 33–36: Final development and monitoring; prepare for labor signs and hospital plan.
- Weeks 37–40: Full-term period begins at 39 weeks; delivery can happen at different times within this range.
How accurate is an LMP-based due date?
LMP dating is very useful, especially with regular cycles and a known start date. Accuracy can be lower if cycles are irregular, if bleeding was unusual, or if ovulation timing was different from average. First-trimester ultrasound is often used clinically to confirm or refine dating.
Tips for using your week-by-week results
- Track your dates and bring them to prenatal appointments.
- Use your current week to plan upcoming screenings and checkups.
- Combine this estimate with your provider’s guidance for medical decisions.
- Remember: estimated due date means an estimate, not an exact appointment date.
Frequently asked questions
Why am I already considered pregnant before conception happened?
Medical gestational age starts at LMP for a standardized timeline. Conception usually occurs about two weeks later in a 28-day cycle.
Can I use this if my cycle is irregular?
Yes, but results may be less precise. Use this as a planning tool and confirm timing with your prenatal provider.
What if I’m past 40 weeks?
Many pregnancies continue beyond 40 weeks. Your care team will monitor you closely and advise next steps based on your and your baby’s health.
Medical note: This page is for educational use and does not diagnose, treat, or replace licensed medical care.