Pregnancy is counted from the first day of your last menstrual period, not from the day you conceived. That means for roughly the first two weeks of your “pregnancy,” you aren’t actually pregnant yet. A full-term pregnancy lasts about 280 days (40 weeks) from that starting point, even though the embryo itself has only been developing for about 266 of those days.
Why Counting Starts Before Conception
This system exists for a practical reason: most people don’t know the exact day they conceived, but they do know when their last period started. Ovulation typically happens about 14 days into a menstrual cycle, and fertilization occurs around that time. Implantation, when the embryo attaches to the uterine wall, happens roughly 9 days after ovulation, with a range of 6 to 12 days. None of those moments come with an obvious signal, so the start of your last period serves as a reliable, consistent reference point.
During “weeks 1 and 2” of pregnancy, your body is going through its normal menstrual cycle. The egg hasn’t been fertilized yet. By the time you miss your next period and get a positive test, you’re already considered about 4 weeks pregnant.
Gestational Age vs. Fetal Age
These two terms describe different clocks. Gestational age counts from the first day of your last period and is the number your doctor uses for everything: scheduling appointments, tracking milestones, and estimating your due date. Fetal age (sometimes called developmental age) counts from the actual moment of conception, which generally falls about 2 weeks after the start of gestation. So if you’re 10 weeks pregnant by gestational age, the embryo has been developing for roughly 8 weeks.
When you read pregnancy apps or week-by-week guides, they almost always use gestational age. The distinction matters most if you’re comparing notes with someone who conceived through IVF, where the conception date is known precisely.
How Your Due Date Is Calculated
The standard formula is called Naegele’s Rule. It works in three steps: take the first day of your last menstrual period, count back 3 calendar months, then add 1 year and 7 days. So if your last period started September 9, 2023, you’d count back to June 9, subtract nothing, then add a year and 7 days to land on June 16, 2024.
This formula assumes a 28-day menstrual cycle with ovulation on day 14. That’s a useful average, but actual pregnancies vary. Research tracking ovulation precisely found the median time from ovulation to birth was 268 days, with a range spanning several weeks in either direction. Even among full-term births, delivery dates ranged from 247 to 284 days after ovulation. Your due date is an estimate, not an appointment.
When Ultrasound Adjusts the Date
A first-trimester ultrasound is the most accurate way to confirm or revise your due date. It measures the embryo from head to rump (called crown-rump length) and is accurate to within about 5 days either way. That’s tighter than second-trimester ultrasound, which has a margin of about 8 days, and considerably better than LMP-based dating alone.
If the ultrasound date and your period-based date are close, your original due date typically stands. If they differ significantly, your provider will usually go with the ultrasound measurement. The American College of Obstetricians and Gynecologists considers first-trimester ultrasound the most accurate method for establishing gestational age and recommends that all pregnancies have dating confirmed by ultrasound before 22 weeks.
If Your Cycles Are Irregular
The LMP method works best when your cycles are predictable and close to 28 days. If your periods are irregular, the gap between your period-based date and your actual gestational age can be substantial. One study of women with irregular cycles found the LMP estimate differed from the ultrasound estimate by more than a week in over half of cases, and by more than two weeks in about a quarter.
Ultrasound dating dramatically improves accuracy for these pregnancies. In that same study, only about 65% of women with irregular cycles delivered within 14 days of their LMP-based due date, compared to nearly 84% when the due date was set by ultrasound. If you don’t remember your last period or your cycles are unpredictable, an early ultrasound becomes especially important for establishing your timeline.
How IVF Pregnancies Are Dated
IVF is a special case because the conception date is known exactly. The formula starts with the transfer date, adds 266 days, and subtracts the age of the embryo at the time of transfer. For a day-5 embryo (blastocyst transfer, the most common type), you count 261 days from the transfer date. For a day-3 embryo, you count 263 days. Your clinic will convert this to a gestational age that matches the standard 40-week system so your prenatal care follows the same schedule as any other pregnancy.
What This Means Week by Week
Understanding the counting system helps make sense of some otherwise confusing math. Here’s what’s actually happening in the early weeks:
- Weeks 1 to 2: Your menstrual cycle is underway. Ovulation happens near the end of this window. You are not yet pregnant.
- Week 3: Fertilization occurs. The fertilized egg travels down the fallopian tube and begins dividing.
- Week 4: Implantation takes place, typically 6 to 12 days after ovulation. Pregnancy hormones start rising. By the end of this week, a home test can usually detect pregnancy.
- Weeks 5 to 6: Most people discover they’re pregnant in this window, often after missing a period. At this point you’re already more than a month into the gestational count.
So when someone says they’re 6 weeks pregnant, the embryo has been developing for about 4 weeks. The two-week offset between gestational age and fetal age stays consistent throughout pregnancy, all the way to delivery at roughly 40 weeks gestational age, or 38 weeks of actual fetal development.