Week 3 Size: Microscopic Blastocyst
Smaller than a grain of salt.
Relative Visual Scale
Solid circle represents your baby's size. Dotted circle shows the full-term week 40 size threshold.
Baby's Development
The fertilized egg (zygote) divides rapidly as it travels down the fallopian tube. It becomes a hollow ball of cells called a blastocyst and begins to implant into the uterine lining.
Weekly Milestones
- ✦ Fertilization
- ✦ Implantation Initiated
Your Body & Symptoms
Implantation triggers the production of hCG (human chorionic gonadotropin), the hormone detected by pregnancy tests. You won't feel this yet, but your body is undergoing a massive shift.
Common Symptoms this Week
Doctor & Care Recommendations
- ✓ Continue taking prenatal vitamins.
- ✓ Keep eating nutrient-dense foods, focusing on leafy greens, lean proteins, and whole grains.
- ✓ Rest when you feel tired; early hormonal changes are starting to drain your energy.
PregWeeks
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Weekly Health Guides & Resources
First Trimester Guide
Read comprehensive clinical milestones, screening checklists, and developmental guidelines for this phase.
Track & Relieve Symptoms
Learn why symptoms occur, home remedies, and clinical indicators for this week:
Food Safety Checklists
Verify safety statuses, listeria and mercury risks, and safe alternatives for:
Frequently Asked Questions: Week 3
Evidence-Based Clinical Sources for Week 3
PregWeeks is dedicated to medical transparency. Every milestone, symptom tip, and fetal development guideline for Week 3 is sourced directly from clinical guidelines published by leading obstetric authorities. Read more about our editorial methodology on our Medical Sources page.
Methods for Estimating Due Date (Committee Opinion No. 721)
ACOGOfficial clinical recommendations on calculating gestational age and due dates using last menstrual period (LMP) and ultrasound measurements.
View Clinical Guideline ↗Folic Acid Guidance to Prevent Neural Tube Defects
CDCPublic health guidelines on the daily recommended intake of prenatal folic acid (400 mcg) before and during early pregnancy.
View Clinical Guideline ↗Antenatal Care for a Positive Pregnancy Experience
WHOGlobal standard guidelines recommending early prenatal visits, nutritional counseling, and basic blood screening starting in the first trimester.
View Clinical Guideline ↗Get weekly pregnancy updates
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