Pregnancy is tracked in weeks starting from the first day of your last menstrual period (LMP), not from the day you conceived. A full pregnancy lasts 40 weeks, or 280 days, counted from that starting point. This means you’re already considered “2 weeks pregnant” at the time conception likely happens, which is why the counting can feel confusing at first.
Why Counting Starts Before Conception
Pregnancy weeks are based on gestational age, which begins on the first day of your last period. Since ovulation and conception typically happen about two weeks into your cycle, gestational age runs roughly two weeks ahead of how long the embryo has actually been developing. Doctors use this system because most people can pinpoint when their last period started, while the exact day of conception is almost never known for certain.
This is why a positive pregnancy test at, say, “4 weeks” doesn’t mean the embryo has been growing for a full month. It means four weeks have passed since your last period began, and the embryo is closer to two weeks old.
How to Calculate Your Due Date
The standard formula, called Naegele’s Rule, works in three steps:
- Step 1: Find the first day of your last menstrual period.
- Step 2: Count back three calendar months from that date.
- Step 3: Add one year and seven days.
For example, if your last period started on March 10, 2025, you’d count back three months to December 10, 2024, then add a year and seven days to get December 17, 2025. That’s your estimated due date.
This formula assumes a 28-day menstrual cycle. If your cycles are consistently longer or shorter, the estimate shifts. Someone with a 35-day cycle ovulates about a week later than the formula expects, which would push the true due date about a week further out. Most pregnancy apps and online calculators let you enter your cycle length to adjust for this.
What to Do With Irregular Cycles
If your periods are unpredictable or you don’t remember when your last one started, an early ultrasound becomes the most reliable way to date the pregnancy. During the first trimester (up through 13 weeks and 6 days), an ultrasound measures the embryo from head to rump. This measurement can pin down gestational age within five to seven days of accuracy, making it more precise than LMP-based calculations for many people.
After the first trimester, ultrasound dating becomes less exact because babies start growing at different rates. That’s why providers prefer to establish your dates early.
How Weeks Map to Trimesters
The three trimesters break down like this:
- First trimester: First day of LMP through 13 weeks and 6 days. This is when major organs form.
- Second trimester: 14 weeks 0 days through 27 weeks and 6 days. The period of rapid growth.
- Third trimester: 28 weeks 0 days through 40 weeks and 6 days. Organs mature and the baby gains weight.
Converting Weeks to Months
Because pregnancy lasts 40 weeks, it actually spans closer to 10 lunar months than the nine calendar months people commonly reference. The week-to-month conversion doesn’t line up neatly, which is why your provider tracks weeks rather than months. But here’s a rough guide:
- Month 1: Weeks 1–4
- Month 2: Weeks 5–8
- Month 3: Weeks 9–13
- Month 4: Weeks 14–17
- Month 5: Weeks 18–21
- Month 6: Weeks 22–26
- Month 7: Weeks 27–30
- Month 8: Weeks 31–35
- Month 9: Weeks 36–40
When someone asks how far along you are, either weeks or months works. Just know that your medical records will always use weeks and days (written as something like “24w3d” for 24 weeks and 3 days).
Milestones That Confirm Your Dates
As your pregnancy progresses, certain checkpoints help confirm that gestational age is on track.
Around week 6, a vaginal ultrasound can detect the first flickers of cardiac activity in the cluster of cells forming the heart. By week 9, a handheld Doppler ultrasound may be able to pick up the heartbeat, and by month 4 it’s loud and clear.
The anatomy scan, typically done around 20 weeks, is the detailed ultrasound that checks physical development and is often when you can learn the sex if you want to. Later in the third trimester, your provider may ask you to do kick counts, tracking how much the baby moves over a set period, as a way to monitor well-being in those final weeks.
Tracking After IVF or Embryo Transfer
If you conceived through IVF, the calculation works a bit differently. Your gestational age is based on the transfer date minus the age of the embryo at the time of transfer. For a 5-day embryo transfer, you’d subtract 5 days from your transfer date to estimate the conception date, then add 266 days to get your due date. A 3-day transfer uses the same logic, subtracting 3 instead. Your fertility clinic will calculate this for you, but understanding the math helps the numbers make sense when you switch to a regular OB provider.
What “Full Term” Actually Means
The definition of a full-term pregnancy is more specific than most people realize. Rather than treating everything from 37 to 42 weeks as the same, medical guidelines now use finer categories. A baby born at 37 weeks through 38 weeks and 6 days is considered early term. Full term runs from 39 weeks through 40 weeks and 6 days. Late term covers 41 weeks through 41 weeks and 6 days, and anything past 42 weeks is postterm.
These distinctions matter because babies born even a couple of weeks early can have different outcomes than those born at 39 or 40 weeks. It’s one reason providers pay close attention to accurate dating throughout pregnancy.
Practical Tips for Tracking
Most people find it easiest to pick one method and stick with it. Pregnancy apps like Ovia, The Bump, and What to Expect let you enter your LMP or due date and will automatically tell you your current week, show size comparisons, and flag upcoming milestones. If your provider adjusts your due date after an early ultrasound, update it in the app so everything stays in sync.
A simple low-tech option: note which day of the week your gestational “birthday” falls on. If your LMP started on a Wednesday, you’ll tick over to a new week every Wednesday. That makes it easy to keep count without checking an app constantly.
Keep in mind that a due date is an estimate, not an appointment. Only about 5% of babies arrive on their exact due date. Most arrive within a window of two weeks on either side, which is why your provider will talk about the due date as a target range rather than a fixed day.