What is Pregnancy Calculator?
A pregnancy calculator estimates your due date and tracks important milestones throughout your pregnancy journey. Whether you've just discovered you're expecting or you're planning ahead, this calculator helps you understand where you are in your pregnancy and what to expect in the coming weeks.
Pregnancy is typically calculated as 40 weeks (280 days) from the first day of your last menstrual period (LMP), not from the date of conception. This is because the exact date of conception is often unknown, but most women can recall when their last period started. The calculator also adjusts the estimate for your cycle length, shows your current trimester and weeks pregnant, and maps key developmental milestones for your baby.
When to Use This Calculator
- After a positive pregnancy test, getting a first estimate of your due date
- Tracking which week and trimester you're in as your pregnancy progresses
- Planning maternity leave start dates and a smooth work handover
- Scheduling baby showers, birth announcements, and family visits around the expected date
- Estimating conception timing when planning or reviewing fertility windows
- Sharing milestone expectations with your partner or healthcare provider
Steps:
- Enter the first day of your last menstrual period.
- Optionally enter your average cycle length (28 days by default).
- View your estimated due date.
- Check your current week of pregnancy and trimester.
- See the weekly milestones you have reached on the chart.
Formula
Due Date = First day of LMP + 280 days (40 weeks)
Alternatively: Due Date = LMP + 7 days - 3 months + 1 year (Naegele's Rule)
With cycle adjustment: Due Date = LMP + 280 + (cycle length − 28) days
Conception date is typically ~2 weeks after LMP (around day 14)
Trimester 1: Weeks 1-12
Trimester 2: Weeks 13-27
Trimester 3: Weeks 28-40+
Use Cases
- Estimating your baby's due date
- Tracking pregnancy progress week by week
- Planning maternity leave and baby preparations
- Understanding developmental milestones during pregnancy
- Checking which trimester you're in before booking appointments or tests
- Estimating a conception window for planning or record-keeping
Key Benefits
- Get a due-date estimate instantly from just your last period and cycle length
- See your current pregnancy week and trimester at a glance
- Understand key developmental milestones with the week-by-week timeline
- Free, private, and works on any device with no downloads or sign-up
- Adjusts the estimate to your own cycle length instead of assuming a fixed 28 days
- Export the timeline as PDF, Excel, or CSV to share with your provider, employer, or family
Pro Tips
- Use the actual first day of your last period, not the day you found out you were pregnant
- If your cycle is longer or shorter than 28 days, enter it — it adjusts the due date by the difference
- Confirm the estimate at your first ultrasound (around 8-12 weeks), which is more accurate than LMP dating
- Expect the baby to arrive anywhere between 37 and 42 weeks, and keep plans flexible
- Share the PDF export with your employer when planning maternity leave
- Recheck the calculator each week to see which milestone your baby has reached
Common Mistakes to Avoid
- Entering a date that isn't the first day of your last period — using a later day shifts the estimate forward
- Assuming the due date is a guarantee — only about 5% of babies arrive exactly on it
- Treating the 40-week count as starting from conception instead of from the last period
- Ignoring cycle length — a 35-day cycle, for example, shifts the estimate about a week later
- Relying on the estimate for medical decisions instead of confirming with an ultrasound and your provider
Key Terms Explained
- LMP (Last Menstrual Period): The first day of your most recent period, the standard starting point for pregnancy dating
- Due Date (EDD): The estimated date of delivery, computed as LMP + 280 days
- Naegele's Rule: The classic formula for estimating the due date from the LMP, assuming a 28-day cycle
- Gestational Week: The age of the pregnancy counted from the first day of the LMP
- Trimester: One of three roughly three-month phases of pregnancy — first (weeks 1-12), second (13-27), third (28-40+)
- Conception Date: The estimated day of fertilization, typically about 14 days after the LMP
Related Concepts
- BMI Calculator — track your weight category before and during pregnancy
- Ideal Weight Calculator — see a healthy weight range for your height
- Calorie Calculator — estimate calories burned during exercise
- Water Intake Calculator — find your daily hydration target
- Baby Cost Calculator — budget for baby gear, care, and nursery essentials
Example
If your last menstrual period started on March 1, 2025, your estimated due date is December 6, 2025 (March 1 + 280 days). On June 1, 2025, you would be approximately 13 weeks pregnant, entering your second trimester. Conception likely occurred around March 15, 2025 (LMP + 14 days).
Interpreting Your Results
Enter the first day of your last menstrual period, and optionally your average cycle length. The calculator adds 280 days to estimate your due date, works out your current week from today, and labels your trimester. The milestone chart shows which weekly developmental milestones you have already reached. Because the method assumes ovulation about 14 days after the LMP, the results are estimates: only about 5% of babies arrive exactly on the due date and most are born between 37 and 42 weeks. Ultrasound dating around 8-12 weeks is more precise, so treat this tool as a planning aid — not as medical advice.