Pregnancy Due Date Calculator (By Weeks)
Use one of the methods below to estimate your baby's due date. You can calculate from your last menstrual period (LMP), conception date, or your current gestational week.
This method adds 40 weeks (280 days), which is the standard obstetric estimate.
This method adds 38 weeks (266 days) from conception.
Example: 21 weeks and 3 days means enter 21 and 3.
Medical note: this tool provides an estimate and is not a diagnosis. Always follow your clinician's guidance.
What is a due date calculator by weeks?
A due date calculator by weeks estimates when your baby is likely to be born based on pregnancy timing. In clinical practice, due dates are usually expressed in gestational age (for example, 16 weeks 2 days) and an estimated due date (EDD).
Most pregnancies are dated from the first day of your last menstrual period, not from the day of conception. That means "week 1" and "week 2" occur before fertilization. This can feel confusing at first, but it is the standard method used by OB-GYNs, midwives, ultrasound departments, and hospitals.
How to use this calculator
1) From LMP (most common)
Enter the first day of your last menstrual period. The calculator adds 280 days (40 weeks) to estimate your due date. This is often called Naegele-style dating and is widely used for initial prenatal planning.
2) From conception or IVF timing
If you know your conception date, the estimate is usually 266 days (38 weeks) from that date. This method is common in fertility treatment planning, where dates can be known more precisely.
3) From current gestational week
If you already know how far along you are (for example, 28 weeks 4 days), enter that gestational age and a reference date (usually today). The calculator then projects how much time is left until week 40.
Why due dates are estimates, not exact predictions
Only a small percentage of babies are born exactly on their due date. Many are born within a window before or after that date. Your due date should be viewed as a planning anchor for prenatal visits, testing milestones, and birth preparation.
- Cycle length differences can shift ovulation timing.
- Implantation timing varies between pregnancies.
- Early ultrasound may refine the dating estimate.
- Natural labor onset differs from person to person.
Week-based pregnancy planning guide
First trimester (weeks 1-13)
- Confirm pregnancy and schedule first prenatal appointment.
- Review medications, supplements, and folic acid intake.
- Discuss early symptoms and warning signs with your provider.
Second trimester (weeks 14-27)
- Energy often improves; continue routine prenatal care.
- Anatomy scan typically occurs around 18-22 weeks.
- Track weight, blood pressure, and fetal movement guidance.
Third trimester (weeks 28-40)
- Increase birth planning, childcare planning, and hospital prep.
- Discuss labor signs, when to call, and postpartum expectations.
- Finalize support systems for after delivery.
Frequently asked questions
Is due date by weeks accurate?
It is usually accurate enough for planning, but not exact for predicting labor day. Think of it as a clinically useful estimate.
Can my due date change?
Yes. Early ultrasound measurements can revise dates if they differ significantly from menstrual dating. Your provider will decide which dating method is most reliable for your case.
What if my cycle is longer or shorter than 28 days?
Atypical cycle lengths can influence ovulation timing, so LMP-only estimates may be less precise. If your cycles are irregular, share that information with your prenatal clinician.
Should I rely only on an online calculator?
No. Use calculators for convenience, then confirm and personalize timing with a healthcare professional.
Final thoughts
A due date calculator by weeks is a practical way to understand where you are in pregnancy and to plan ahead. Use this tool to estimate your timeline, then keep your prenatal care team at the center of decision-making. Your provider can incorporate ultrasound findings, cycle history, and medical context to give you the best timeline for your pregnancy.