Interactive Due Date Week-by-Week Calculator
Use your last menstrual period (LMP) or conception date to estimate your due date, current pregnancy week, and a full week-by-week calendar.
How this due date week by week calculator works
This tool uses standard obstetric dating to estimate your expected due date and show pregnancy progress one week at a time. Most providers date pregnancy from the first day of your last menstrual period (LMP), even though conception usually happens about two weeks later.
- LMP method: Due date = LMP + 280 days (40 weeks), adjusted by cycle length.
- Conception method: Due date = conception date + 266 days (38 weeks).
- Week-by-week timeline: Displays each pregnancy week, date range, trimester, and milestones.
How to use it
1) Choose your method
If you know your period start date, choose LMP. If conception is known (for example, tracked ovulation or fertility treatment), choose conception date.
2) Enter dates carefully
Small date shifts can change your estimated due date by several days. If you have irregular cycles, use the conception method if available for a more specific estimate.
3) Review your week-by-week schedule
After calculating, scroll through your personalized pregnancy calendar to see date ranges for every week from week 1 through week 42.
Understanding pregnancy weeks and trimesters
Pregnancy is generally divided into three trimesters:
- First trimester: Weeks 1-13
- Second trimester: Weeks 14-27
- Third trimester: Weeks 28-42
Not all pregnancies deliver exactly at 40 weeks. Full-term birth typically occurs between 39 and 40 weeks, and birth between 37 and 42 weeks can still be normal depending on clinical context.
Typical week-by-week milestones
Every pregnancy is unique, but these are common milestones often discussed during prenatal care:
- Week 4-5: Pregnancy test may become positive.
- Week 6-8: Early ultrasound may detect heartbeat.
- Week 10-13: End of first trimester approaches.
- Week 18-22: Anatomy ultrasound is commonly scheduled.
- Week 24-28: Glucose screening often performed.
- Week 28+: Third trimester monitoring increases.
- Week 37+: Early-term period begins.
- Week 39-40: Full-term window and expected due date period.
How accurate is an estimated due date?
A due date is an estimate, not a deadline. Many people deliver before or after that day. First-trimester ultrasound can sometimes provide a more accurate clinical estimate than menstrual dating, especially if cycles are irregular or LMP is uncertain.
Important medical note
This calculator is for educational planning only and does not replace medical advice. Always confirm dating and prenatal care plans with your OB-GYN, midwife, or healthcare provider. Seek urgent care for heavy bleeding, severe pain, fluid leakage, or reduced fetal movement in later pregnancy.
FAQ
Can my due date change after an ultrasound?
Yes. Providers may update the estimated due date if ultrasound measurements differ significantly from LMP-based dating.
What if I don’t know my exact LMP date?
Use your best estimate and discuss uncertainty with your provider. Ultrasound can help refine timing.
Why does cycle length matter?
If your cycle is longer or shorter than 28 days, ovulation may occur later or earlier, so adjusting cycle length can improve the estimate.