Following your pregnancy timeline on a weekly schedule offers excellent insights into your physical changes, baby growth stages, and clinical check schedules. Tracking gestational age helps you prepare for regular scans and keep checklists organized. Use our free online Pregnancy Week-by-Week Calculator to trace your calendar.
How to Use the Pregnancy Week-by-Week Calculator
- Input biological markers: Select the first day of your last menstrual period (LMP) and provide your average cycle length.
- Optional known details: Enter your expected due date if it has already been established by a dating scan.
- Review your outputs: Click Calculate to review weeks and days gestation, progress percentage, trimester landmarks, and baby size metrics.
Formula & Calculation Method
Pregnancy week calculation uses the first day of the last menstrual period as the starting baseline:
Days Pregnant = Current Date - LMP Date Current Week = floor(Days Pregnant ÷ 7) + 1 Days in Week = Days Pregnant % 7 Expected Due Date = LMP Date + 280 Days + (Cycle Length - 28 Days)
Trimester transitions are estimated standardly (First: W1-13, Second: W14-27, Third: W28-40), and baby size metrics map to established fetal growth charts.
Example Calculation
If your Last Menstrual Period began on April 1, 2026, and your cycle is 28 days:
- Gestation Weeks: On August 19, 2026 (140 days pregnant), you are exactly 20 weeks 0 days along.
- Trimester Boundary: You are currently at the midway point of the Second Trimester.
- Due Date Estimate: January 6, 2027 (April 1 + 280 days).
- Fetal Size Indicator: Baby is about the size of a banana, weighing ~10.6 oz and measuring ~10 inches.
Reference Data: Gestational Weeks Milestone & Development
| Gestational Week | Trimester | Fetal Development Highlights | Typical Maternal Physical Signs |
|---|---|---|---|
| Week 4 | First | Implantation completes; neural tube begins forming. | Mild cramping, light spotting, early fatigue. |
| Week 8 | First | Heartbeat detectable; fingers and toes start separating. | Morning sickness, frequent urination, breast changes. |
| Week 12 | First | Most major organs formed; vocal cords start to develop. | Nausea begins to settle; energy levels start to return. |
| Week 16 | Second | Fetal gender visible; eyes can move slowly. | Growing belly bump; round ligament stretching sensations. |
| Week 20 | Second | Halfway mark; first movements (quickening) felt. | Increased appetite; sleep adjustments needed. |
| Week 24 | Second | Taste buds formed; skin begins storing fat reserves. | Braxton Hicks contractions; skin stretching/itching. |
| Week 28 | Third | Baby can open eyes; lungs develop surfactant. | Backaches, mild shortness of breath, leg cramps. |
| Week 32 | Third | Skeleton fully formed; baby gains weight rapidly. | Pelvic pressure, sleep difficulties, mild heartburn. |
| Week 36 | Third | Lungs near maturity; baby drops into vertex position. | Frequent contractions, nesting instinct, pelvic heaviness. |
| Week 40 | Third | Baby is fully mature and ready for delivery. | Cervical effacement, loss of mucus plug, labor signs. |
What the Result Means
Calculations establish progress ranges to match standard obstetric care models. Each weekly step connects baby development highlights with common maternal body shifts, giving you checklist prompts for tests like glucose screens and anatomy scans.
Common Mistakes to Avoid
- Confusing conception week with gestational age: Standard calendars date pregnancy starting from LMP, which means you are technically "0 weeks pregnant" before conception even occurs.
- Expecting perfect milestones matching: Biological progress has variation. If your baby measures slightly ahead or behind size indicators, it is usually a normal variant.
- Overlooking irregular cycle lengths: If cycles regularly vary from the standard 28-day model, count calculations starting from dating scans instead of LMP.
When This Calculator Is Useful
Weekly Growth Comparisons
Track baby growth markers using checkable fruit and vegetable comparisons to visualize weekly development.
Trimester Progress Tracking
Identify boundary changes as you transition between trimesters to anticipate physical changes.
OB-GYN Visit Planning
Prepare checklist items for standard scans, diabetic screens, and strep tests ahead of clinical appointments.
Maternity Leave Schedules
Align countdown numbers with your career timelines to prepare maternity leave plans with confidence.
This tool provides estimates based on standard clinical calculations. Always consult your midwife or OB-GYN for accurate dating and medical prenatal support.