The clock starts at your last period
Pregnancy weeks are counted from the first day of your last menstrual period, not from conception. In a typical 28-day cycle, ovulation and conception happen around day 14. So by the time an egg is fertilized, you're already counted as 2 weeks pregnant.
That sounds odd, but it's practical. Most people know when their period started. Very few know the day they conceived. Starting from a date you can actually name gives everyone, you and your care team, the same reference point.
What 14 weeks and 1 day means
Gestational age is written as completed weeks plus extra days. 14 weeks 1 day means 99 days have passed since the start of the clock, and you're in your 15th week. The days count from 0 to 6, then roll over into the next week.
This is where people trip up. "I'm in week 15" and "I'm 14 weeks" describe the same day. Apps and books switch between the two, so check which one they mean before comparing.
Formula notes
Gestational age in days = check date - first day of last periodWeeks = days / 7, rounded down; extra days = the remainder
Weeks, trimesters and months
Trimesters split the 40 weeks into three parts. Following ACOG's patient guidance, the first runs through 13 weeks 6 days, the second from 14 weeks 0 days through 27 weeks 6 days, and the third from 28 weeks on.
Months are where counting gets fuzzy. Calendar months run 28 to 31 days, but four weeks is always 28 days. Forty weeks is only 280 days, a little over nine calendar months. That's why your care team talks in weeks.
- First trimester: 0 weeks 0 days to 13 weeks 6 days
- Second trimester: 14 weeks 0 days to 27 weeks 6 days
- Third trimester: 28 weeks 0 days until birth
- Full term: 39 weeks 0 days to 40 weeks 6 days
Worked cases
Say your last period started April 1, 2026. On May 6, 2026, that's 35 days later, so you're exactly 5 weeks 0 days. On July 9 it's 99 days, or 14 weeks 1 day, and you've just entered the second trimester. Your estimated due date is 280 days after April 1: January 6, 2027.
If your cycles run 32 days, ovulation is probably later than day 14. Shifting the start by four days moves the due date to January 10, 2027 and makes July 9 only 13 weeks 4 days. Our pregnancy calculator applies that cycle adjustment for you, and it can start from a conception date, an ultrasound or an IVF transfer too.
Why your due date might change, and when it shouldn't
Your first due date usually comes from your last period. If an early ultrasound measures a different age, your care team may move the date. ACOG calls a first-trimester ultrasound the most accurate way to confirm gestational age, and for IVF it recommends dating from the transfer.
After the date is set, it normally stays put. Later scans estimate the baby's size, and size varies. ACOG says changes to an established due date affect care and should be discussed and documented. Keep the date in your record, even if an app gives you a slightly different one.
And remember the due date is a midpoint. ACOG notes only about 1 in 20 women give birth on that exact day. Full term covers two weeks, from 39 weeks 0 days through 40 weeks 6 days.
Frequently asked questions
Am I pregnant during the first two weeks?
Usually not. Those two weeks cover your period and the days before ovulation. They're counted so the clock can start on a date you know.
What week of pregnancy is a missed period?
With a 28-day cycle, your period is due 28 days after the last one started, which is 4 weeks 0 days on the gestational clock.
How far along am I counting from conception?
Subtract about two weeks from your gestational age. A pregnancy at 10 weeks gestational age is roughly eight weeks past conception, assuming ovulation near day 14.
Why do apps disagree on what week I'm in?
Some show completed weeks and others show the week you're currently in. At 14 weeks 1 day, one app may say 14 and another week 15.
How many days is a full pregnancy?
The standard estimate is 280 days, or 40 weeks, from the first day of your last period. From conception it's about 266 days.
References
These sources support the method or guidance used for Pregnancy Calculator. Verify time-sensitive rules at the source.
Try the calculator
Open Pregnancy Calculator, enter your scenario, and compare its supporting rows with this guide's method and checks.