Pregnancy Week Calculator
Track your babys development week by week. Calculate your current pregnancy week, see baby size comparisons, and discover key milestones.
Understanding Pregnancy Development
Pregnancy is a miraculous 40-week journey of growth and development. Every week brings new changes for both you and your baby. Our pregnancy week calculator helps you track this incredible transformation with accurate week calculations and developmental milestones.
How Pregnancy Weeks Are Calculated
Pregnancy weeks are counted from the first day of your last menstrual period (LMP), not from conception. This method is used because:
- The LMP date is more easily remembered and accurate
- It provides a standard 40-week timeline
- All medical appointments and tests are scheduled based on LMP dating
- Conception usually occurs around week 2 of this timeline
The Three Trimesters
| Trimester | Weeks | Key Developments | Common Changes |
|---|---|---|---|
| First | 1-12 | Organ formation, heartbeat begins | Morning sickness, fatigue, breast tenderness |
| Second | 13-28 | Rapid growth, movement begins | Energy returns, baby bump shows |
| Third | 29-40 | Lung maturation, weight gain | Increased discomfort, nesting instinct |
Baby Development Milestones
Your babys development follows a predictable timeline with amazing milestones:
First Trimester Highlights
- Week 4: Neural tube forms, heart begins developing
- Week 5: Heart starts beating (detectable by ultrasound)
- Week 8: All major organs present, officially called a fetus
- Week 12: Reflexes develop, sex organs form
Second Trimester Highlights
- Week 16: Baby can hear, facial expressions develop
- Week 18: Sex can be determined via ultrasound
- Week 20: Halfway point! Anatomy scan typically performed
- Week 24: Viability milestone - baby could potentially survive outside the womb with medical support
Third Trimester Highlights
- Week 28: Eyes can open, brain activity increases
- Week 32: Bones harden (except skull), rapid weight gain
- Week 36: Lungs nearly mature, baby is considered late preterm if born
- Week 37: Full-term begins! Baby is ready for birth
Baby Size Comparisons
Comparing your babys size to familiar objects helps visualize their growth:
- 8 weeks: Raspberry (1.6cm) - Major organs formed
- 12 weeks: Lime (5cm) - End of first trimester
- 20 weeks: Banana (16.5cm) - Halfway milestone
- 28 weeks: Eggplant (37.5cm) - Third trimester begins
- 36 weeks: Honeydew melon (46.5cm) - Nearly full-term
- 40 weeks: Small pumpkin (51.5cm) - Full-term
Weight Gain Recommendations
Healthy weight gain varies by pre-pregnancy BMI:
| Pre-pregnancy BMI | Total Weight Gain | Weekly Gain (2nd/3rd tri) |
|---|---|---|
| Underweight (<18.5) | 28-40 lbs (12.5-18 kg) | 1-1.3 lbs/week |
| Normal (18.5-24.9) | 25-35 lbs (11-16 kg) | 0.8-1 lb/week |
| Overweight (25-29.9) | 15-25 lbs (7-11 kg) | 0.5-0.7 lbs/week |
| Obese (>30) | 11-20 lbs (5-9 kg) | 0.4-0.6 lbs/week |
Important Prenatal Care Timeline
- Weeks 6-8: First prenatal visit, confirm pregnancy
- Weeks 10-13: First trimester screening (optional)
- Weeks 15-20: Maternal serum screening, amniocentesis (if indicated)
- Weeks 18-22: Anatomy ultrasound scan
- Weeks 24-28: Glucose tolerance test for gestational diabetes
- Weeks 35-37: Group B strep test
When to Contact Your Healthcare Provider
Contact your doctor immediately if you experience:
- Severe abdominal pain or cramping
- Heavy bleeding or spotting
- Severe headaches or vision changes
- Persistent vomiting preventing food/fluid intake
- Decreased fetal movement after 28 weeks
- Signs of preterm labor before 37 weeks
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Frequently Asked Questions
How are pregnancy weeks calculated?
Pregnancy weeks are calculated from the first day of your last menstrual period (LMP), not from conception. A full-term pregnancy is 40 weeks (280 days) from the LMP date.
When does my baby start moving?
Your baby starts moving around 7-8 weeks, but you typically feel first movements (quickening) between 16-25 weeks. First-time mothers usually feel movement later.
How big is my baby at 20 weeks?
At 20 weeks, your baby is about the size of a banana - approximately 16.5 cm long and weighs around 300 grams. This is the halfway point of pregnancy!
What are the three trimesters?
First trimester: weeks 1-12 (organ formation), Second trimester: weeks 13-28 (rapid growth), Third trimester: weeks 29-40 (preparation for birth).
When is my baby considered full-term?
Your baby is considered full-term between 37-42 weeks. Early term is 37-39 weeks, full term is 39-41 weeks, and late term is 41-42 weeks.
How much weight should I gain?
Weight gain depends on your pre-pregnancy BMI. Generally: 25-35 lbs for normal weight, 28-40 lbs for underweight, 15-25 lbs for overweight, and 11-20 lbs for obese women.
When can I find out my babys gender?
Gender can typically be determined via ultrasound around 15-20 weeks, with accuracy increasing after 18 weeks. Blood tests can detect gender as early as 10 weeks.
What is the anatomy scan?
The anatomy scan is a detailed ultrasound performed around 18-22 weeks to check your babys development, organs, and detect any potential abnormalities.
When does morning sickness end?
Morning sickness usually peaks around 8-11 weeks and typically improves by 12-14 weeks (end of first trimester), though some women experience it longer.
How often should I have prenatal appointments?
Typical schedule: every 4 weeks until 28 weeks, every 2 weeks from 28-36 weeks, then weekly until delivery. High-risk pregnancies may require more frequent visits.
When does brain development begin?
Brain development starts very early - the neural tube forms around week 3-4, and basic brain structure is established by week 6. Rapid brain growth continues throughout pregnancy.
What are Braxton Hicks contractions?
Braxton Hicks are "practice" contractions that can start around 20 weeks. They are irregular, painless tightening of the uterus, unlike real labor contractions which are regular and painful.