Pregnancy weeks are counted from the first day of your last menstrual period (LMP), not from the day you actually conceived. This means that during “week one” and “week two” of pregnancy, you aren’t pregnant yet. A full-term pregnancy lasts about 40 weeks by this counting method, even though the embryo has only been developing for roughly 38 of those weeks.
This system can feel counterintuitive, but there’s a practical reason behind it: most people know when their last period started, while the exact date of conception is nearly impossible to pin down. Ovulation timing varies, and sperm can survive in the body for several days. The first day of your period, on the other hand, is a concrete, observable date.
The LMP Method Step by Step
To figure out how far along you are, start with the first day of your most recent period. Count forward from that date to today, and that number of weeks is your gestational age. If your last period started on March 1 and today is April 12, you’re about six weeks pregnant.
This same starting point is used to estimate your due date. The standard formula, known as Naegele’s rule, works like this: take the first day of your last period, count back three calendar months, then add one year and seven days. So if your last period started on June 10, you’d count back to March 10 and then add a year and seven days, giving you a due date of March 17 the following year.
Naegele’s rule assumes a 28-day menstrual cycle. If your cycle is consistently shorter or longer, your ovulation likely happened earlier or later than the standard day-14 estimate, which shifts your true conception date and potentially your due date by several days. The NHS recommends adjusting the calculation based on your actual cycle length when possible.
Gestational Age vs. Fetal Age
You’ll sometimes see two different “ages” mentioned in pregnancy resources, and they refer to different things. Gestational age is the one your doctor uses. It counts from the first day of your last period. Fetal age (also called embryonic age) counts from the estimated date of conception, which typically happens about two weeks after the start of your last period. So fetal age always runs roughly two weeks behind gestational age.
Medical professionals use gestational age almost exclusively because it provides a more consistent reference point. Fetal development markers vary from pregnancy to pregnancy. Not every embryo hits the same milestone at the same time, and physical or genetic factors can shift the pace of development. The LMP-based system gives everyone a shared timeline to work from, even if the actual biology is a little messier.
When Ultrasound Overrides the Math
An early ultrasound is the most accurate way to confirm how far along you are. In the first trimester (up to about 14 weeks), a measurement of the embryo from head to tailbone gives a gestational age estimate that’s accurate to within five to seven days. The earlier in the first trimester the scan is done, the more precise it is.
If the ultrasound date and your LMP-based date are close, your provider will generally stick with the LMP date. But if there’s a meaningful gap between the two, the ultrasound measurement takes priority. This matters because an accurate due date affects how your provider interprets growth, schedules certain tests, and determines whether your pregnancy has gone past term.
Ultrasound dating becomes especially important if you have irregular periods, don’t remember when your last period started, or were using hormonal birth control that may have affected your cycle. In these situations, the LMP method can be off by weeks, and an early scan is the best way to establish a reliable timeline.
How Trimesters Break Down by Week
The three trimesters divide the 40-week timeline into distinct phases, each defined by specific week-and-day thresholds:
- First trimester: First day of LMP through 13 weeks and 6 days. This is when fertilization occurs and the major organs begin forming.
- Second trimester: 14 weeks and 0 days through 27 weeks and 6 days. A period of rapid growth.
- Third trimester: 28 weeks and 0 days through 40 weeks and 6 days. The fetus gains weight and organs mature in preparation for birth.
A pregnancy is considered full-term anywhere from 37 weeks to 42 weeks. That five-week window is normal, which is why a “due date” is really more of a due range.
Counting Weeks After IVF
IVF pregnancies use a slightly different calculation because the transfer date is known precisely. For a day-3 embryo transfer, you add 263 days to the transfer date to estimate your due date. For a day-5 transfer, you add 261 days. The two-day difference accounts for the extra development time the embryo had in the lab before being transferred.
To convert an IVF due date into a gestational age that matches the standard LMP system, providers calculate backward to find a theoretical “last menstrual period” date. From that point on, your pregnancy weeks are tracked identically to any other pregnancy. This means your 12-week ultrasound, your 20-week anatomy scan, and all the other time-based milestones will line up on the same schedule.
Why the Numbers Can Feel Confusing
One common source of confusion is that pregnancy is described as “nine months” in everyday conversation but tracked as 40 weeks medically. Forty weeks is actually closer to ten calendar months. The discrepancy exists because months vary in length (28 to 31 days), while weeks are always seven days. The weekly system is simply more precise.
Another stumbling block: when your provider says you’re “8 weeks pregnant,” that means you’ve completed eight full weeks. You’re currently in your ninth week. It works the same way as age. A one-year-old has finished their first year and is living in their second. So “8 weeks pregnant” and “in your ninth week” mean the same thing.
If you’re ever unsure where you fall, the simplest approach is to count the number of complete weeks since the first day of your last period. That number is your gestational age, and it’s what every prenatal chart, test schedule, and milestone guide is built around.