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?
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.
The Three Trimesters: A Roadmap
Pregnancy is a marathon divided into three distinct legs, each lasting roughly 12–13 weeks.
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, your baby is about 3 inches long—roughly the size of a lemon!
2. Second Trimester (Weeks 14–26)
The Golden Period. For many, the nausea fades and energy returns.
Milestones: You will likely start to feel “quickening” (baby’s movements). By week 20, the baby weighs about 10 ounces. This is also when the anatomy scan usually happens.
3. Third Trimester (Weeks 27–40+)
The Home Stretch. It’s all about growth and preparation.
Milestones: By week 32, the baby can open and close their eyes. 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 1: Weeks 1 to 4
Month 2: Weeks 5 to 8
Month 3: Weeks 9 to 13
Month 4: Weeks 14 to 17
Month 5: Weeks 18 to 21
Month 6: Weeks 22 to 26
Month 7: Weeks 27 to 30
Month 8: Weeks 31 to 35
Month 9: Weeks 36 to 40+
Conclusion
Calculating your due date is more of an art than a science. Only about 5% of babies are born exactly on their due date! 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.
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