Calculating How Many Weeks Pregnant You Are
Pregnancy is counted from the first day of your last menstrual period (LMP), not from conception. This means you're technically "pregnant" for about 2 weeks before you actually conceived. It's confusing, but it's the standard medical convention worldwide.
Weeks pregnant = (Today's date - First day of last period) / 7
For example, if your last period started exactly 56 days ago, you are 8 weeks pregnant (56 / 7 = 8).
Pregnancy Weeks vs. Months
Converting weeks to months isn't straightforward because months aren't exactly 4 weeks. Here's the standard medical breakdown:
| Month | Weeks | Trimester |
|---|---|---|
| Month 1 | 1-4 | First |
| Month 2 | 5-8 | First |
| Month 3 | 9-13 | First |
| Month 4 | 14-17 | Second |
| Month 5 | 18-22 | Second |
| Month 6 | 23-27 | Second |
| Month 7 | 28-31 | Third |
| Month 8 | 32-35 | Third |
| Month 9 | 36-40 | Third |
Week-by-Week Milestones
| Week | Baby's Size | Key Development |
|---|---|---|
| 4 | Poppy seed | Implantation complete, positive pregnancy test possible |
| 6 | Lentil | Heart begins beating, neural tube forming |
| 8 | Raspberry | Fingers and toes forming, facial features developing |
| 10 | Strawberry | All major organs formed, now called a fetus |
| 12 | Lime | Reflexes developing, risk of miscarriage drops significantly |
| 16 | Avocado | Can make facial expressions, sex may be visible on ultrasound |
| 20 | Banana | Halfway point! Anatomy scan, movements felt ("quickening") |
| 24 | Corn on the cob | Viability milestone, lungs developing surfactant |
| 28 | Eggplant | Eyes can open, regular sleep/wake cycles, third trimester begins |
| 32 | Squash | Bones hardening, practicing breathing, gaining weight rapidly |
| 36 | Honeydew melon | Lungs nearly mature, dropping into birth position |
| 40 | Watermelon | Full term! Average weight 7.5 lbs, average length 20 inches |
Common Questions About Pregnancy Timing
Why do doctors count from the last period and not conception?
Because most women know when their last period started but don't know the exact date of conception. Even if you know when you had intercourse, sperm can survive up to 5 days, so conception could have happened days later. The LMP provides a consistent starting point.
I know my conception date. How do I convert to weeks?
Add 2 weeks to your conception-based count to match the LMP-based system. If you conceived 10 weeks ago, you are considered 12 weeks pregnant in medical terms.
Can I be further along than my LMP suggests?
Yes. If you have short cycles (e.g., 24 days), you likely ovulated earlier than day 14, meaning you may be slightly further along. An early ultrasound can clarify your exact gestational age.
What does "6 weeks 3 days" mean?
Pregnancy is measured in complete weeks plus remaining days. "6 weeks 3 days" (written 6w3d) means 6 full weeks and 3 additional days have passed since your LMP — a total of 45 days.