How this pregnancy calculator works
Pregnancy is counted from the first day of the last menstrual period (LMP), about two weeks before conception, and lasts 280 days or 40 weeks on average. This pregnancy calculator turns whichever date you know into that starting day, then counts forward to today to show how far along you are and lays out the full schedule week by week.
| Calculate from | Start of pregnancy (day 0) |
|---|---|
| Due date | Due date − 280 days |
| Last period | First day of the last period + (cycle length − 28) days |
| Ultrasound | Ultrasound date − gestational age that day |
| Conception date | Conception date − 14 days |
| IVF transfer date | Transfer date − embryo age (3, 5 or 6 days) − 14 days |
The estimated due date is always day 0 plus 280 days. If your cycle is longer or shorter than 28 days, ovulation is assumed to shift by the same number of days, which moves the whole schedule with it.
What the results include
- Current week. Week 1 covers days 1 through 7, so at 22 weeks 6 days you are in week 23.
- Weeks pregnant. The completed weeks and extra days since day 0, which is the gestational age your provider uses, along with the same span in calendar months.
- Trimester. The first trimester runs through week 12, the second through week 27, and the third from week 28 on.
- Likely conception date. Shown when you start from a due date, last period or ultrasound: 266 days before the due date.
- Baby's size. The average length and weight for the current week, from week 9 through week 40.
- Week-by-week schedule. The calendar dates of all 42 weeks with common milestones, and today highlighted.
From 37 weeks the results also name the term stage: early term from 37 weeks 0 days, full term from 39 weeks 0 days, late term from 41 weeks 0 days and postterm from 42 weeks 0 days.
Frequently asked questions
How far along am I, and how many weeks pregnant?
Count the days from the first day of your last period to today and divide by 7. The whole number is your completed weeks and the remainder is the extra days. Choose Last period above and the calculator does this for you.
Why does pregnancy start before conception?
The first day of the last period is a date most people know, while the day of conception usually is not. Dating from the period adds about two weeks, which is why a 40-week pregnancy is about 38 weeks from conception.
How many months is 40 weeks?
About nine calendar months and one week. Months vary in length, so the calculator counts whole calendar months from day 0 and then the leftover days, rather than dividing weeks by four.
Will my baby be born on the due date?
Probably not on the exact day. A due date marks 40 weeks, and a birth from 37 weeks on is considered term. Treat the date as the middle of a window rather than a deadline.
Which starting point is the most accurate?
An early ultrasound usually dates a pregnancy best, and an IVF transfer date is exact. A last period date works well when your cycles are regular and you are sure of the date.
Limitations
This calculator gives general estimates for a single pregnancy and is not medical advice. Size figures are averages, and healthy babies vary. Twins and other multiples are usually delivered earlier. Your obstetric provider's dating takes priority over any estimate here.
What's changed
- Launched.
References
- ACOG Committee Opinion No. 700, Methods for Estimating the Due Date
280 days from the last period, ultrasound dating, and dating by embryo age and transfer date
- ACOG Committee Opinion No. 579, Definition of Term Pregnancy
early term, full term, late term and postterm ranges