How Many Weeks Pregnant Am I? Gestational Age Explained

How the Weeks Are Counted

Gestational age, the standard measure used in all pregnancy care, is counted from the first day of your last menstrual period, abbreviated LMP. Week one begins on that day. A full-term pregnancy runs to 40 weeks, which is 280 days from the LMP, and this is where your due date comes from.

So when a midwife says you’re “10 weeks,” they mean 10 weeks have passed since the start of your last period. The count is written in weeks and days, such as “10 weeks and 3 days,” because those extra days matter for tracking development and timing tests.

Why It Starts Before Conception

Here’s the part that confuses almost everyone. Because the count starts at your last period, and ovulation and conception happen roughly two weeks later, you’re considered about two weeks pregnant at the moment you conceive, and around four weeks pregnant by the time a missed period prompts a test.

The reason for this apparently odd convention is practical. The first day of a period is a date most people can identify, while the exact day of conception usually isn’t known. Counting from the LMP gives a consistent, usable starting point. It does mean gestational age runs about two weeks ahead of the embryo’s actual age, sometimes called fetal or conceptional age, which is why two sources can quote ages a fortnight apart. Our guide on how a due date is calculated covers this in more detail.

Working Out Your Week

To find your current week, count the days from the first day of your last period to today, then divide by seven.

Example: if your last period began 1 June and today is 4 August, that’s 64 days. Divide by seven and you get 9 weeks and 1 day. So you’d be described as 9 weeks and 1 day pregnant, or in your tenth week. Note the wording: being “9 weeks” means you’ve completed nine weeks and are into the tenth, which is another common source of confusion.

This assumes a roughly average cycle. If your cycles are much longer or shorter than 28 days, the LMP count can be off by several days, which a dating scan later corrects.

The Three Trimesters

Pregnancy is divided into three stretches, each with its own developments.

The exact week boundaries between trimesters vary slightly between sources, but the framework is consistent. Reaching 37 weeks matters because a birth from then is considered term rather than premature, and 39 to 40 weeks is full term.

How a Scan Adjusts the Count

The LMP count is an estimate, and an early ultrasound gives a more accurate one. A dating scan in the first trimester measures the embryo and can establish gestational age to within about five to seven days, which is more accurate than the period-based count, especially if your cycle is irregular or your LMP date is uncertain.

If the scan disagrees with the LMP count by more than roughly five to seven days, your provider will usually revise your dates and due date to match the scan. This is a normal part of care, not a sign anything is wrong, and it’s why your “weeks pregnant” number can shift slightly after your first scan. From that point, the revised dates guide the rest of your pregnancy.

Frequently Asked Questions

How do I know how many weeks pregnant I am?

Count the days from the first day of your last period to today and divide by seven. That gives your gestational age in weeks and days. A calculator does it instantly, and an early dating scan gives a more precise figure, particularly if your cycle is irregular.

Why am I two weeks pregnant when I conceived?

Because pregnancy is counted from the first day of your last period, about two weeks before ovulation and conception. This convention uses a date you can identify rather than the usually unknown day of conception, so gestational age runs roughly two weeks ahead of the embryo’s actual age.

What are the three trimesters?

The first runs from weeks 1 to 13, when organs form and early symptoms appear; the second from 14 to 27, often the most comfortable, with the anatomy scan around 18 to 22 weeks; and the third from 28 to 40, with rapid growth and preparation for birth. Term is reached from 37 weeks.

How many weeks is a full-term pregnancy?

Forty weeks from the last period is the standard length and the basis for the due date. A birth from 37 weeks is considered term, 39 to 40 weeks is full term, and beyond 42 weeks is post-term. Only a small share of babies actually arrive exactly on the due date.

Does my week change after a scan?

It can. An early dating scan measures the embryo and is accurate to within about five to seven days. If it differs from your period-based count by more than that, your provider revises your dates and due date to match the scan. This is routine and not a cause for concern.

What if my periods are irregular?

Then the period-based count is less reliable, and an early ultrasound becomes the better way to date the pregnancy. This is common after stopping hormonal contraception or with conditions affecting the cycle, and your provider will use the scan to establish your weeks and due date.

Does “9 weeks pregnant” mean I’ve completed 9 weeks?

Yes. Being described as 9 weeks means nine full weeks have passed and you’re in your tenth week, similar to how a nine-year-old has completed nine years. The weeks-and-days format, such as “9 weeks and 3 days,” removes the ambiguity for medical purposes.

The Bottom Line

How many weeks pregnant you are is counted from the first day of your last period, which is why you’re already about two weeks along at conception and four weeks by a missed period. Count the days since your LMP and divide by seven for your gestational age, remember the three trimesters run 1 to 13, 14 to 27, and 28 to 40 weeks, and expect an early scan to fine-tune the figure. Work out your week and due date with the pregnancy due date calculator, and let your midwife or doctor confirm your dates at your first scan.

Sources and Further Reading

• American College of Obstetricians and Gynecologists. “Methods for Estimating the Due Date.”
• UK NHS — guidance on pregnancy weeks, trimesters, and dating scans.
• World Health Organization — definitions of term and preterm birth.