To figure out how far along you are, count the number of weeks from the first day of your last menstrual period (LMP) to today. A full-term pregnancy is about 40 weeks, or 280 days, counted from that date. This is the same method your doctor uses, and it means your “pregnancy clock” actually starts roughly two weeks before conception, since ovulation typically happens around day 14 of your cycle.
The Standard Way to Count Pregnancy Weeks
Gestational age is measured from the first day of your last period, not from the day you conceived. That distinction confuses a lot of people, because it means you’re technically “two weeks pregnant” at the time of conception. Doctors use this system because most people can recall when their period started, while pinpointing the exact day of conception is much harder.
To estimate your due date, you can use a simple formula called Naegele’s Rule: take the first day of your last period, count back three calendar months, then add one year and seven days. If your last period started June 1, you’d count back to March 1 and add a year and seven days, giving you a due date of March 8 the following year. This formula assumes a 28-day cycle. If your cycles are longer or shorter, the estimate shifts.
When You Know Your Conception Date
If you know the date you conceived (or a close approximation), you can calculate your due date by adding 266 days, or about 38 weeks, from that date. Research tracking pregnancies from ovulation found the median time from ovulation to birth is 268 days (38 weeks and 2 days), with a standard deviation of about 10 days. That means most deliveries fall within a roughly three-week window around the due date, not on a single magic day.
To convert from conception-based counting to the gestational age your doctor uses, simply add two weeks. So if you conceived four weeks ago, you’re considered six weeks pregnant in medical terms.
How Ultrasound Dates Your Pregnancy
Between 9 and 13 weeks, an ultrasound measuring the length of the embryo from head to rump is the most accurate way to date a pregnancy. At this stage, growth is rapid and consistent across pregnancies, so the margin of error is roughly plus or minus a day or two. Later in pregnancy, the fetus curls up and individual size differences become more pronounced, making ultrasound dating less precise.
If your ultrasound date and your period-based date are within a week of each other, most providers stick with the period-based date. If they differ by more than a week, the ultrasound measurement typically overrides it, since it’s considered more reliable.
Why Irregular Cycles Change the Math
The 28-day cycle that Naegele’s Rule assumes doesn’t apply to everyone. If your cycles are irregular, counting from your last period can be significantly off. In one study of women with irregular periods, more than half had a difference of over seven days between their period-based due date and the one calculated by ultrasound. A quarter were off by more than two weeks.
Ultrasound dating dramatically improved accuracy for these women. When using only their last period, about 20% appeared to be overdue at delivery. When ultrasound measurements were used instead, that number dropped to 2.5%. If your cycles are longer than 35 days, shorter than 21, or unpredictable, an early ultrasound is especially important for getting an accurate timeline.
Dating an IVF Pregnancy
If you conceived through IVF or another assisted reproduction method, the calculation is more straightforward because the transfer date is known. Your conception date equals the transfer date minus the age of the embryo at transfer. For a day-5 blastocyst transfer, you’d subtract five days from the transfer date to get the conception date, then add 266 days for your due date. To convert to gestational age in weeks, add two weeks to the time since conception, just as with a natural pregnancy.
Weeks, Months, and Trimesters
Pregnancy weeks map to trimesters like this:
- First trimester: week 1 through the end of week 12
- Second trimester: week 13 through the end of week 26
- Third trimester: week 27 until delivery
Converting weeks to months is less tidy than you’d expect, because calendar months aren’t all the same length. As a rough guide, four weeks equals about one month, so 16 weeks is roughly four months and 28 weeks is roughly seven months. But when your provider discusses your progress, they’ll almost always use weeks, because months are too imprecise. “32 weeks” tells a provider something very different from “34 weeks,” even though both fall in “month eight.”
Can a Blood Test Tell You How Far Along You Are?
A blood test measuring hCG (the hormone your body produces during pregnancy) can confirm pregnancy earlier than a home test, but it’s not a reliable way to pinpoint how many weeks along you are. The reason is the enormous range of normal values at any given week. At five weeks, for example, hCG can be anywhere from 18 to 7,340 mIU/mL. At seven to eight weeks, the range spans from 7,650 to 229,000. Two people at the exact same gestational age can have wildly different levels, so a single number can’t tell you your week with any confidence. Serial blood draws showing the rate of rise can confirm a pregnancy is progressing normally, but for dating purposes, ultrasound is far more accurate.
How Precise Is Any Due Date?
Even with perfect dating, natural variation in pregnancy length is real. Research tracking pregnancies from confirmed ovulation found a standard deviation of 10 days, meaning the middle 50% of deliveries fell within a range of about two and a half weeks. Only about 4% of babies arrive on their calculated due date. Think of your due date as the center of a window rather than a target. Most full-term babies arrive between 39 and 41 weeks, and that entire range is considered normal.