If you just missed your period and got a positive pregnancy test, you are already about 4 weeks pregnant. That number surprises most people, but it’s because pregnancy isn’t counted from the day you conceived. It’s counted from the first day of your last menstrual period, which is roughly two weeks before conception even happened.
Why You’re Already 4 Weeks at a Missed Period
Doctors measure pregnancy in “gestational age,” which starts on the first day of your last period, not the day sperm met egg. Since ovulation and conception typically happen around two weeks into your cycle, there’s a built-in two-week gap between your gestational age and how long the embryo has actually been developing. So when your period is one week late, you’re considered 5 weeks pregnant, even though the embryo is closer to 3 weeks old.
This system exists because most people can reliably remember when their last period started, while pinpointing the exact day of conception is nearly impossible. Healthcare providers use gestational age in weeks (not months) to plan every aspect of prenatal care, from blood tests to anatomy scans.
How to Count Your Weeks
Start with the first day of your last menstrual period. That date is Day 1 of your pregnancy. From there, count forward in weeks to today’s date. If your last period started on March 1 and today is April 5, you’re 5 weeks pregnant.
To estimate your due date, you can use a formula called Naegele’s Rule, which Johns Hopkins Medicine describes in three steps:
- Step 1: Find the first day of your last menstrual period.
- Step 2: Count back 3 calendar months from that date.
- Step 3: Add 1 year and 7 days.
So if your last period started June 10, 2025, count back three months to March 10, then add one year and seven days: your estimated due date is March 17, 2026. This formula assumes a 28-day cycle, and most pregnancies last about 40 weeks (280 days) from that starting point.
What If Your Cycles Are Irregular
The 28-day cycle is just a baseline. If your cycles are consistently longer or shorter, your ovulation day shifts, which means the standard count may be off. A person with a 35-day cycle likely ovulated around day 21 rather than day 14, so they’d be about a week less far along than a simple calculation suggests.
If you tracked your average cycle length before getting pregnant, you can use that number to adjust. Many online due date calculators let you enter your typical cycle length and will correct the estimate for you. But if your cycles were unpredictable, an early ultrasound becomes the most reliable way to date the pregnancy.
When an Ultrasound Changes Your Due Date
An ultrasound in the first trimester measures the embryo’s size, which correlates closely with gestational age during those early weeks. According to guidelines from the American College of Obstetricians and Gynecologists, if an ultrasound before 9 weeks shows a due date that differs from your period-based estimate by more than 5 days, your provider will likely change the due date to match the ultrasound. Between 9 and 14 weeks, the threshold widens to 7 days.
This matters because an accurate due date affects the timing of screening tests, determines whether growth is on track later in pregnancy, and guides decisions about labor if you go past your due date. If your dates get adjusted at an early ultrasound, use the new date going forward when counting your weeks.
IVF and Known Conception Dates
If you conceived through IVF, dating is more precise because the transfer date is known. For a 5-day embryo transfer, the conception date is considered 5 days before the transfer. For a 3-day transfer, it’s 3 days before. From that conception date, add 266 days to find your due date. Your clinic will convert this into a gestational age that matches the standard weekly system, so your prenatal care stays on the same timeline as any other pregnancy.
What’s Happening in These Early Weeks
Understanding where you fall on the timeline also helps you picture what’s developing. At 4 weeks (the point you miss your period), the embryo has just implanted into the uterine lining and is smaller than a grain of rice. The hormone that triggers a positive pregnancy test is rising rapidly, from roughly 5 to 426 mIU/mL during week 4.
By week 5, the neural tube that will become the brain and spinal cord is forming, and a tiny cluster of cells that will become the heart begins pulsing, reaching about 110 beats per minute by the end of that week. At week 6, small buds that will grow into arms and legs appear, and early structures for the eyes, ears, and mouth take shape. A vaginal ultrasound can often detect cardiac activity at this point.
Hormone levels climb steeply during this stretch: by weeks 7 to 8, they can range from 7,650 to 229,000 mIU/mL. This rapid rise is a big reason nausea and fatigue tend to hit hardest in the first trimester.
Weeks vs. Months: A Quick Translation
People outside the medical world often think in months, but pregnancy months don’t line up neatly with calendar months because most calendar months are longer than four weeks. Here’s a rough guide:
- Weeks 1 to 4: Month 1 (you find out at the very end of this stretch)
- Weeks 5 to 8: Month 2
- Weeks 9 to 13: Month 3, end of the first trimester
- Weeks 14 to 27: Second trimester
- Weeks 28 to 40: Third trimester
When someone asks how far along you are, giving the answer in weeks is the most precise and the number your provider will use at every appointment. If your last period started 6 weeks ago, you’re 6 weeks pregnant, in your second month, and right at the beginning of some of the most active early development.