So, you saw those two pink lines. Congratulations! But after the initial excitement settles, the confusion often sets in. You are 6 weeks pregnant, but only 1 month along? And the doctor says you are in your first trimester, but you haven’t had a bump yet?
A quick note: This article is for general information and isn’t medical advice. Every pregnancy is different. Talk to your doctor or midwife about your own situation, and get urgent medical help if something feels wrong. Read our disclaimer.
Last reviewed: November 2026. Checked against guidance from ACOG and the NHS. See sources.
Pregnancy math can be surprisingly complicated. It isn’t as simple as 9 months on the calendar.
In this guide, we will break down exactly how doctors calculate your due date, why we count from before you were even pregnant, and how to translate those confusing weeks into months.
How is Pregnancy Actually Calculated?
The most common question is: If I conceived two weeks ago, why am I considered four weeks pregnant?
The “LMP” Method
Doctors calculate pregnancy starting from the First Day of your Last Menstrual Period (LMP).
- Why? It is very difficult to pinpoint the exact moment of conception (fertilization). However, most women know when their last period started.
- The Math: The calculation assumes a standard 28-day cycle where ovulation happens around day 14. So, the first two “weeks” of your pregnancy are actually the weeks leading up to ovulation. You aren’t technically pregnant yet, but these weeks count toward the 40-week total.
Your Due Date Formula: First day of LMP + 280 days (40 weeks) = Estimated Due Date
Note: If your cycle is irregular, your doctor may adjust this date based on an early ultrasound, which measures the baby’s size to determine gestational age more accurately.
Want your exact week, trimester and due date? Try our free pregnancy due date calculator. Trying to conceive? The ovulation calculator shows your likely fertile days.
The Three Trimesters: A Roadmap
Pregnancy is a marathon divided into three distinct legs. ACOG counts the first trimester from the first day of your last period to 13 weeks and 6 days, the second from 14 weeks to 27 weeks and 6 days, and the third from 28 weeks onwards (ACOG).
1. First Trimester (Weeks 1–13)
The Invisible Work. Even if you aren’t showing yet, your body is working harder than ever.
- Milestones: At week 4, the fertilized egg implants. By week 8, major organs are forming. By the end of this trimester, the kidneys have started making urine and fingernails are forming (ACOG).
2. Second Trimester (Weeks 14–27)
The Golden Period. For many, the nausea fades and energy returns.
- Milestones: You will likely start to feel “quickening” (baby’s movements). Most people feel first movements between 16 and 24 weeks (NHS). The anatomy (20-week) scan is usually done between 18 and 21 weeks (NHS).
3. Third Trimester (Weeks 28–40+)
The Home Stretch. It’s all about growth and preparation.
- Milestones: By about weeks 25 to 28, the baby’s eyelids can open and close, and by weeks 29 to 32 the eyes can sense changes in light (ACOG). As you approach week 40, the baby gains weight rapidly to regulate body temperature after birth, and your body prepares for labor.
The Translation Guide: Weeks to Months
Because pregnancy is 40 weeks, it is actually closer to 10 lunar months (28 days) than 9 calendar months. This is why the “month” breakdown can feel tricky.
Here is the cheat sheet to help you answer when people ask, “How many months are you?”
| Month of pregnancy | Weeks | Trimester |
|---|---|---|
| Month 1 | Weeks 1–4 | First |
| Month 2 | Weeks 5–8 | First |
| Month 3 | Weeks 9–13 | First |
| Month 4 | Weeks 14–17 | Second |
| Month 5 | Weeks 18–21 | Second |
| Month 6 | Weeks 22–26 | Second |
| Month 7 | Weeks 27–30 | Second, then third from week 28 |
| Month 8 | Weeks 31–35 | Third |
| Month 9 | Weeks 36–40+ | Third |
Conclusion
Calculating your due date is more of an art than a science. Only about 1 in 20 women give birth on their due date (ACOG), and the NHS says pregnancy normally lasts from 37 to 42 weeks from the first day of your last period (NHS). Whether you are tracking by weeks or months, remember that every pregnancy is unique. Trust your healthcare provider, listen to your body, and enjoy the countdown.
Sources
We checked this article against these official and expert sources. Links were checked on 1 November 2026.
- ACOG. How Your Fetus Grows During Pregnancy
- NHS. Due date calculator
- NHS. Your baby’s movements
- NHS. 20-week scan
Last reviewed: November 2026. This article is for general information and isn’t medical advice. Always talk to your doctor about your own situation.
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