Pregnancy is dated from the first day of your last menstrual period (LMP), not from the day you actually conceived. By this standard convention, a full-term pregnancy lasts 280 days, or 40 weeks. That means your “week count” starts about two weeks before conception even happened, which surprises many people but is how every doctor’s office, ultrasound report, and due date calculator works.
There are several ways to figure out where you are in pregnancy, ranging from a simple calendar count to an ultrasound measurement. Here’s how each method works and how reliable it is.
Counting From Your Last Period
The quickest way to estimate how far along you are is to count the weeks and days since the first day of your most recent period. If that date was exactly 8 weeks ago, you’re considered 8 weeks pregnant, even though the embryo itself is closer to 6 weeks old. This two-week gap exists because the clock starts at menstruation, not ovulation or fertilization.
To estimate your due date using this method, add 280 days (40 weeks) to that first day. So if your last period started on January 1, your estimated due date would be October 8. Dozens of free online calculators do this math instantly. Just keep in mind that this method assumes a regular 28-day cycle with ovulation around day 14. If your cycles are longer, shorter, or unpredictable, the estimate can be off by a week or more.
Why Irregular Cycles Change the Math
If your periods are irregular, dating by LMP alone is significantly less accurate. A study of 277 women with irregular cycles found that in more than half the cases, the LMP-based estimate and the ultrasound-based estimate differed by more than 7 days. In about one in four women, the gap exceeded two full weeks. When researchers compared both methods against the actual delivery date, ultrasound dating was substantially better at predicting when the baby would arrive. Among women with irregular periods, LMP dating classified 20% of pregnancies as “post-term,” while ultrasound dating dropped that figure to just 2.5%.
If you don’t remember the exact start date of your last period, or your cycles vary widely, an early ultrasound is the most reliable way to pin down your dates.
How Ultrasound Dating Works
In the first trimester, an ultrasound measures the length of the embryo from head to rump, called the crown-rump length. This measurement is remarkably consistent across pregnancies of the same age, which makes it a strong dating tool. At 9 weeks, the average measurement is about 27 millimeters. By 11 weeks it’s roughly 49 mm, and by 13 weeks it reaches about 73 mm.
First-trimester ultrasounds are the most accurate for dating, generally reliable to within about five to seven days. As pregnancy progresses, babies start growing at more individual rates, so second- and third-trimester ultrasounds become less precise for pinpointing gestational age. That’s why an early scan, ideally between 8 and 13 weeks, gives the best estimate. If your ultrasound date and your LMP date are close, your provider will typically stick with the LMP. If they diverge by more than a week in the first trimester, the ultrasound date usually takes priority.
If You Conceived Through IVF
When conception happens through IVF or another assisted reproduction method, dating is more straightforward because the exact transfer date is known. The calculation works backward from the transfer: subtract the age of the embryo at transfer (3 days for a Day 3 embryo, 5 days for a Day 5 embryo) to find the conception date, then add 266 days for the due date. This removes the guesswork entirely and is considered the most precise method of pregnancy dating available.
Gestational Age vs. Fetal Age
You’ll hear two different age references during pregnancy, and they can be confusing. Gestational age is the number your provider uses. It counts from your last period and is always about two weeks ahead of the embryo’s actual age. Fetal age (sometimes called conceptional age) counts from the moment of fertilization. So when you’re told you’re “6 weeks pregnant,” the embryo is roughly 4 weeks old. Every milestone chart, app notification, and medical guideline uses gestational age, so that’s the number to track.
What Your Symptoms Can Tell You
Symptoms aren’t precise enough to date a pregnancy, but they can offer rough clues if you’re trying to piece together a timeline before your first appointment. Light spotting from implantation typically shows up 10 to 14 days after conception, which would place it around week 4 of gestational age. Morning sickness commonly begins one to two months in, so roughly weeks 4 through 8. Breast tenderness from hormonal shifts is one of the earliest signs and can start within the first few weeks, though it varies widely from person to person.
None of these symptoms appear on a reliable schedule, and some women experience all of them while others notice almost none. They’re useful for a ballpark guess, not a date on the calendar.
What HCG Levels Don’t Tell You
You might wonder whether the pregnancy hormone level in your blood can reveal exactly how far along you are. It can’t. The hormone becomes detectable in blood about 8 to 10 days after ovulation and roughly doubles every 1.4 to 2.1 days in early pregnancy, reaching its peak around 8 to 10 weeks. But the normal range at any given week is enormous, so a single blood draw can confirm pregnancy and track whether levels are rising appropriately, but it cannot pinpoint a specific week of gestation.
Fundal Height in Later Pregnancy
Once you’re past the halfway point, your provider has another simple tool: measuring the distance from your pubic bone to the top of your uterus with a tape measure. After week 24, this measurement in centimeters roughly matches the number of weeks you are, give or take about 3 centimeters. At 30 weeks, for example, the measurement should be somewhere around 27 to 33 centimeters. It’s not a dating tool for early pregnancy, but it helps confirm that growth is on track later on.
Putting It All Together
If you’re sitting at home right now trying to figure out how far along you are, start with the simplest approach: count the weeks since the first day of your last period. If that date is clear and your cycles are regular, that number is probably accurate within a week. If your cycles are irregular, you conceived through fertility treatment, or you’re unsure of the date, an early ultrasound will give you the most reliable answer. Your provider will reconcile all available information, your LMP, any ultrasound measurements, and your cycle history, to assign a gestational age and due date that guides the rest of your care.