How due dates are estimated
The standard obstetric formula is Naegele's rule: due date = first day of last menstrual period (LMP) + 280 days. That's 40 weeks, measured from LMP rather than from conception (a quirk of how pregnancies are dated - you're considered "2 weeks pregnant" on the day you actually conceive).
We adjust this for non-28-day cycles. If your cycle is 32 days, you ovulate around day 18 instead of day 14, so conception happens 4 days later - and your due date shifts forward by 4 days.
Trimester milestones
- First trimester: weeks 1–13. Organ formation, early symptoms, highest miscarriage risk.
- Second trimester: weeks 14–27. Usually the easiest stretch - energy returns, bump becomes visible, you may feel the first kicks around week 18–22.
- Third trimester: weeks 28–40+. Rapid baby growth, more frequent appointments, prep for delivery.
How accurate is the due date?
Only about 5% of babies are born on their exact due date. Most arrive within 2 weeks before or after. Full term covers a wide window: 37 to 42 weeks. Early ultrasound (especially the dating scan around 8–13 weeks) is the most accurate way to confirm or revise the LMP-based estimate.
Frequently asked questions
How is my due date calculated?
We use Naegele's rule - the standard obstetric formula. Take the first day of your last menstrual period (LMP), add 280 days (40 weeks), and that's your estimated due date. If your cycle isn't 28 days, we adjust: longer cycles ovulate later, so your due date shifts forward; shorter cycles shift it back.
How accurate is a due date calculator?
An LMP-based estimate is accurate to within about a week for regular cycles. Only about 5% of babies arrive on their exact due date - most are born within 2 weeks before or after. Early ultrasound (especially in the first trimester) is the most accurate way to confirm a due date and may revise the LMP estimate.
What if I don't know the date of my last period?
If you don't know your LMP, an ultrasound in the first trimester can date the pregnancy by measuring the fetus. You can also work backward from a known conception date (add about 266 days). If you tracked ovulation, conception likely occurred on or within 1 day of ovulation.
Does cycle length affect the due date?
Yes. The standard Naegele's rule assumes a 28-day cycle. Longer cycles (e.g. 32 days) ovulate around day 18 instead of day 14, so conception is later and the due date shifts forward by the difference. We apply this adjustment automatically when you set your cycle length.
When does pregnancy officially start - at conception or LMP?
Pregnancy is counted from the first day of your last menstrual period (LMP), not from conception. This is a quirk of obstetric convention - it means you're already considered 2 weeks pregnant on the day you actually conceive. By the time you miss a period and get a positive test, you're already 4 weeks along.
Is my due date the latest I can be pregnant?
No. Due date is the midpoint of a normal delivery window. Full-term pregnancy is anywhere from 37 to 42 weeks. About 60% of births happen within a week of the estimated due date, and most providers wait until at least 41 weeks before considering induction unless there are medical reasons to deliver earlier.
These calculators give estimates based on cycle averages and standard formulas. They are for general information only and are not medical advice. For anything concerning your health or pregnancy, talk to a qualified healthcare provider.