Pregnancy Week by Week: What's Happening Inside (And What You Should Be Doing)
You're pregnant — congratulations! Now what? Here's a clear week-by-week guide to fetal development, what to expect, and what to do at each stage.
The Pregnancy Timeline
Pregnancy lasts approximately 40 weeks (280 days) from the first day of your last period. That's about 9 months and 1 week. Here's what happens in each trimester.
First Trimester (Weeks 1-12): The Foundation
| Week | Baby's Development | What You Might Feel |
|---|---|---|
| 4 | Heart starts beating | Missed period, positive test |
| 6 | Brain and spine forming | Morning sickness begins |
| 8 | Fingers and toes forming | Fatigue, frequent urination |
| 10 | All organs formed | Morning sickness peaks |
| 12 | Reflexes developing | Nausea starts to ease |
Second Trimester (Weeks 13-27): The Golden Period
| Week | Baby's Development | What You Might Feel |
|---|---|---|
| 16 | Skeletal system hardens | Energy returns, baby bump appears |
| 20 | Halfway point! Can hear sounds | First kicks ("quickening") |
| 24 | Lungs developing | Growing belly, back pain |
| 27 | Eyes can open | Braxton Hicks contractions |
Third Trimester (Weeks 28-40): The Home Stretch
| Week | Baby's Development | What You Might Feel |
|---|---|---|
| 32 | Practicing breathing | Shortness of breath, heartburn |
| 36 | Gaining 1oz per day | Frequent urination returns |
| 38 | Fully developed | Nesting instinct, contractions |
| 40 | Ready to meet you! | Labor signs |
Step-by-Step: Calculate Your Due Date
- Open the pregnancy calculator
- Enter the first day of your last period
- Get your estimated due date, current trimester, and week-by-week milestones
- Save or screenshot for your reference
Try it now: Calculate Your Due Date →
See your current week, trimester, and upcoming milestones.
Frequently Asked Questions
How is pregnancy due date calculated?
Due date is calculated as 280 days (40 weeks) from the first day of your last menstrual period (LMP). This is Naegele's rule. Actual delivery happens within 2 weeks of the due date for 80% of women.
What are the three trimesters of pregnancy?
First trimester (weeks 1-12): organ development, morning sickness. Second trimester (weeks 13-27): growth, energy returns. Third trimester (weeks 28-40): rapid weight gain, preparation for birth.
When should I start prenatal care?
Schedule your first prenatal visit as soon as you get a positive test (around weeks 6-8). Early prenatal care reduces the risk of complications by 30-50%.