Pregnancy Calculator: Due Date, Week-by-Week Progress, and Trimesters
A pregnancy calculator estimates your due date and tracks progress in weeks. The math is based on conventions used by obstetricians worldwide. This guide explains what the numbers mean and how to use them.
Standard pregnancy is 280 days (40 weeks) from the first day of the last menstrual period (LMP) β the start of the most recent period before conception. This is the convention used in OB/GYN clinics globally, even though actual conception happens about 14 days later (around ovulation).
Counting from LMP is preferred because it's a date most people know, while exact conception dates are usually unknown. The 'gestational age' you'll hear in clinic is measured from LMP. So a fetus described as '12 weeks gestational' is actually about 10 weeks since conception.
Pregnancy timeline at a glance
| Period | Weeks | Notes |
|---|---|---|
| First trimester | 1β12 | Most miscarriages happen here. Major organ formation. |
| Second trimester | 13β27 | Often most comfortable phase. 'Quickening' (first felt movement) ~16β22 wks. |
| Third trimester | 28β40 | Major weight gain. Movement very noticeable. |
| Full term | 37β42 | Babies born here are not considered premature. |
| Due date (EDD) | 40 | Naegele's rule: LMP + 280 days. |
| Post-term | 42+ | Most providers induce by 41β42 weeks if not delivered. |
How the due date is calculated (Naegele's rule)
EDD (Estimated Date of Delivery) = LMP + 280 days. Equivalently: take LMP, subtract 3 months, add 7 days, add 1 year. The rule was developed in the 1800s and remains the standard.
Only about 4% of babies are actually born on their EDD. About 60% are born within a week before or after, and 90% within two weeks. Variability is normal β first pregnancies tend to go slightly past the due date on average.
If you know your conception date or use IVF, the calculation can be more precise: EDD = conception date + 266 days (or transfer date + 261 days for day-5 IVF transfers).
What to do with these numbers
- β’Schedule prenatal care at recommended weeks: first ultrasound around 8β12 weeks; anatomy scan around 18β22 weeks; gestational diabetes screening around 24β28 weeks.
- β’Plan maternity leave timing β typical paid leave starts a few weeks before due date in countries that offer it.
- β’Track viability milestones: 24 weeks is the typical 'survivability' threshold with modern NICU care.
- β’Don't fixate on the EDD itself β use it as a planning anchor, but expect Β±2 weeks variability in actual birth date.
Extended FAQ
What if I don't know my LMP?
An early ultrasound can establish dating accurately to within a few days when done in the first trimester. By the second trimester, dating ultrasounds become less precise.
How do I count weeks correctly?
If you're 8 weeks 3 days pregnant, you've completed 8 full weeks plus 3 days. The day after is 8 weeks 4 days. After 7 days you've completed another full week.
Does irregular cycle change the calculation?
Yes β the standard formula assumes a 28-day cycle. If yours is significantly longer or shorter, the LMP-based estimate may be off by a week or more, and an ultrasound becomes more important for accurate dating.
Is this calculator a substitute for prenatal care?
No β it's only a date helper. All prenatal monitoring requires an obstetrician or midwife.
Are my dates stored?
No β runs entirely in your browser.