Pregnancy Trimesters
| Trimester | Weeks | Key Events |
|---|---|---|
| First | Weeks 1–13 | Implantation, organ formation, heartbeat detectable (week 6–8) |
| Second | Weeks 14–26 | Fetal movement felt, gender visible on ultrasound, anatomy scan |
| Third | Weeks 27–40 | Rapid growth, lung maturation, head engagement, birth |
Key Pregnancy Milestones
| Week | Development Milestone |
|---|---|
| Week 4 | Embryo implants; positive pregnancy test possible |
| Week 6 | Heartbeat visible on transvaginal ultrasound |
| Week 8 | All major organs present; embryo now called fetus |
| Week 10 | Fingers and toes formed; NIPT screening available |
| Week 12 | End of highest-risk period; nuchal scan; 1st trimester screening |
| Week 16–20 | Anatomy ultrasound; fetal movement usually felt (quickening) |
| Week 20 | Halfway point |
| Week 24 | Viability threshold — survival possible with intensive NICU care |
| Week 28 | Third trimester begins; RhoGam shot if Rh-negative |
| Week 37 | Full term (previously 39; guidelines updated 2013) |
| Week 40 | Due date (EDD) |
| Week 42 | Post-term; induction typically recommended |
Frequently Asked Questions
Why does pregnancy count from the last period, not conception?
Conception usually occurs about 2 weeks after the first day of the last menstrual period (LMP). Counting from LMP standardizes dating because the exact day of conception is rarely known, while the LMP date is usually remembered. This means you are technically already "2 weeks pregnant" at the time of conception.
How accurate is the due date?
The estimated due date (EDD) is accurate to within ±2 weeks for most pregnancies. Only about 5% of babies are born on their exact due date. Early ultrasound (before 13 weeks) provides the most accurate dating and may adjust the EDD if it differs significantly from the LMP-based estimate.
What if my cycle is not 28 days?
The standard 280-day (40-week) calculation assumes a 28-day cycle with ovulation at Day 14. For longer cycles, your EDD may be later; for shorter cycles, earlier. Your OB or midwife will use ultrasound measurements (especially crown-rump length before week 14) to confirm or adjust your due date.