Pregnancy Due Date Calculator (Week by Week)
Enter the first day of your last menstrual period (LMP) and your average cycle length to estimate your due date, current gestational age, and a full 40-week timeline.
Educational use only. This tool does not replace care from your OB-GYN, midwife, or physician.
How this due date calculator works
Most due date tools use a standard method called Naegele’s rule. It starts with the first day of your last period and adds 280 days (40 weeks). Because cycle length can vary, this calculator also adjusts by the difference between your cycle and a 28-day cycle.
Formula used
- Estimated due date: LMP + 280 days + (cycle length − 28)
- Estimated conception date: LMP + 14 days + (cycle length − 28)
- Gestational age: Today − LMP, shown in weeks and days
Pregnancy is usually dated from LMP, not conception, so week 1 and week 2 occur before fertilization in many cycles. That can feel strange at first, but it is the standard medical dating approach.
Week-by-week pregnancy timeline at a glance
The table generated above shows all 40 weeks with date ranges and milestone notes. This makes it easy to plan scans, prenatal appointments, travel, and leave.
First trimester (weeks 1–13)
- Hormones rise quickly and early symptoms can begin.
- Major organ systems start forming.
- Around week 6 to 8, cardiac activity may be seen on ultrasound.
- By week 12 to 13, risk of miscarriage generally declines compared with very early weeks.
Second trimester (weeks 14–27)
- Energy often improves for many people.
- The detailed anatomy scan is commonly done around week 20.
- Movement may become easier to notice between weeks 18 and 22.
- Growth accelerates and body changes become more visible.
Third trimester (weeks 28–40)
- Weight gain and fetal growth continue steadily.
- Breathing practice, brain development, and fat storage increase.
- Week 37 is considered early term; week 39 is full term.
- Labor can start before, on, or after the estimated due date.
What can change your due date estimate?
A due date is an estimate, not an exact prediction. Only a small percentage of babies arrive exactly on that date. Factors that can shift dating include:
- Irregular cycle length or uncertain LMP date
- Late or early ovulation
- Early ultrasound measurements that refine gestational age
- Assisted reproduction timing (IUI/IVF)
If your provider updates your due date after an ultrasound, follow their clinical dating. Medical teams combine cycle data and imaging to choose the most accurate timeline.
How to use your week-by-week result
Plan appointments and testing
Use the generated timeline to estimate when you may schedule first prenatal visits, screening options, anatomy scan windows, and third-trimester checkups.
Prepare for practical milestones
Your weekly dates can help with family planning, work leave discussions, childcare setup, and travel decisions. It is especially useful to track the “term window” (about week 37 through week 42), when spontaneous labor can occur.
Track progress without stress
Weekly progress is helpful, but remember that every pregnancy is unique. The biggest value of a due date calculator is orientation and planning, not precision to a single day.
Frequently asked questions
Is my due date exact?
No. Think of it as the center of a range. Birth can naturally occur several weeks before or after the estimated date.
Why is pregnancy counted from my period and not conception?
Because conception date is often uncertain in natural cycles. LMP gives a standardized starting point used worldwide.
What if I do not know my LMP?
An early ultrasound is usually the best way to establish pregnancy dating when LMP is unclear or cycles are irregular.
Should I rely on online calculators only?
No. Use calculators for education and planning, then confirm with your healthcare professional for medical decisions.
Final note
A due date calculator week by week is one of the simplest tools for understanding pregnancy timing. Use it to get organized, reduce uncertainty, and have better conversations with your care team.