Third Trimester Start Calculator

How it is calculated

The tool counts in days, not weeks. The third trimester is treated as starting at day 196 (28 weeks × 7), so the calculation is days_until = 196 − days_pregnant, and weeks_until = days_until ÷ 7. The only input is days_pregnant, the number of days since the first day of the last menstrual period (or the dating-scan equivalent). The day count is rounded to a whole number and the weeks to one decimal. If you are already at or past day 196 the result is simply 0, not a negative number. Values below 0 or above 310 days return no result. Some sources and clinics draw the trimester boundaries slightly differently, so treat 28 weeks as the convention used here.

Common mistakes

• Entering weeks instead of days. Typing 17 for a 17-week pregnancy tells the tool you are 17 days pregnant and gives 179 days remaining; multiply weeks by 7 and add the extra days.
• Entering the week number you are currently in. Being in week 17 means you have completed 16 weeks, which is 112 days or more, not 119.
• Counting from conception rather than from the last menstrual period. Gestational age is dated from the period, which is about two weeks earlier than conception.
• Expecting a negative number once you are past 28 weeks. The result stops at 0, so 0 means the third trimester has already begun.
• Entering a value above 310 days, which returns no result.