Pregnancy Weeks Calculator
Use this calculator to estimate how many weeks pregnant you are based on the first day of your last menstrual period (LMP), plus your estimated due date.
Important: This tool gives an estimate and does not replace medical care. Always confirm dating and due date with your clinician and ultrasound findings.
How a pregnancy weeks calculator works
Most pregnancy dating starts from your LMP (last menstrual period), not the date of conception. That means gestational age includes about two weeks before ovulation for people with a typical cycle.
In practical terms, if this calculator says you are 10 weeks pregnant, the embryo/fetus is usually about 8 weeks from conception. This can sound confusing, but it is the standard method used in obstetric care.
What this calculator estimates
- Current gestational age in weeks and days
- Week of pregnancy (for example, 22 weeks and 3 days means you are in week 23)
- Estimated due date (EDD) based on LMP and cycle length
- Approximate conception date based on cycle assumptions
- Time remaining until due date (or days overdue)
- Trimester and rough progress through a 40-week pregnancy
Input guide for better accuracy
1) First day of your LMP
Enter the first day your last period started. If you are unsure of the exact date, use your best estimate and discuss confirmation dating scans with your provider.
2) Cycle length
A 28-day cycle is the default. If your cycle is usually shorter or longer, adjusting this field can improve the estimate. The due date is shifted from the standard 280 days accordingly.
3) Reference date
This is the date you want to calculate for. By default, the calculator uses today’s date, but you can check future appointments or past dates as needed.
Pregnancy weeks by trimester
- First trimester: Week 1 to week 13
- Second trimester: Week 14 to week 27
- Third trimester: Week 28 to birth
Milestones, test timing, and symptom patterns are often discussed by trimester, so knowing your exact week can be very useful during prenatal visits.
Weeks to months: a quick reference
Months are not equal in length, so conversion is approximate. A practical guide:
- Weeks 1–4: Month 1
- Weeks 5–8: Month 2
- Weeks 9–13: Month 3
- Weeks 14–17: Month 4
- Weeks 18–22: Month 5
- Weeks 23–27: Month 6
- Weeks 28–31: Month 7
- Weeks 32–35: Month 8
- Weeks 36–40: Month 9
Why your due date may change
Early ultrasound can refine gestational age, especially if your cycles are irregular or if LMP is uncertain. It is common for clinicians to adjust estimated due date after first-trimester measurements.
Remember: only a small percentage of babies are born exactly on the due date. Think of EDD as a useful target, not a guarantee.
Frequently asked questions
Is this calculator accurate for irregular cycles?
It can still provide a rough estimate, but irregular ovulation can shift actual conception timing. For irregular cycles, ultrasound-based dating is usually more reliable.
Can I use conception date instead of LMP?
Clinical dating is typically LMP-based, then adjusted by ultrasound if needed. If you only know conception date, your provider can help map that to gestational age.
What if my result says I am overdue?
Going past 40 weeks can happen. Contact your care team for guidance on monitoring and planning. Never rely only on online tools if you have symptoms or concerns.
Final note
A pregnancy calculator by weeks is a practical planning tool for appointments, testing windows, and personal milestones. Use it as a guide, but let your prenatal care team provide final medical dating and advice.