This is the week it happens. A single sperm meets the egg in the fallopian tube, and fertilization occurs. Within hours, the fertilized egg begins dividing. By the end of the week, this tiny cluster of cells is making its way toward the uterus. Use our Due Date Calculator to find your estimated delivery date.
Key Takeaways
- Fertilization occurs in the fallopian tube, producing a single-celled zygote that immediately begins dividing.
- By day 5–6, the zygote has developed into a blastocyst of roughly 100 cells and begins traveling toward the uterus for implantation.
- The baby's sex is genetically determined at the moment of fertilization — X chromosome from sperm produces a girl (XX); Y chromosome produces a boy (XY).
- No home pregnancy test will show positive this week — hCG levels are still too low to detect.
Your Baby This Week
Size: Smaller than the tip of a pen (0.1 to 0.2 mm)
Right after fertilization, the single cell (called a zygote) starts dividing rapidly. By day 3, it has become a ball of about 16 cells called a morula. By day 5 or 6, it has developed into a blastocyst, a hollow ball of roughly 100 cells with an inner cell mass that will become your baby and an outer layer that will become the placenta.
The blastocyst travels down the fallopian tube and reaches the uterus, where it will begin to implant into the uterine lining. Track your baby's growth with our Milestone Tracker.
Your Body
You will not feel any different yet. It is far too early for pregnancy symptoms. A home pregnancy test will not show a positive result this week because the hormone hCG has not started circulating in significant amounts.
Behind the scenes, progesterone from the corpus luteum (the empty follicle left after ovulation) is keeping the uterine lining thick and receptive.
What to Do This Week
- Keep taking folic acid. Neural tube development starts very early, and adequate folate is essential. Read our fetal development guide for a trimester-by-trimester overview of what happens next.
- Avoid known risks. If you are trying to conceive, steer clear of alcohol, smoking, and high-mercury fish.
- Be patient. There is nothing to test or confirm yet. The two-week wait between ovulation and a missed period is the hardest part for many people.
Common Questions
When does implantation happen?
Implantation typically occurs 6 to 12 days after ovulation. Most commonly, it happens around day 9. Some people experience light spotting or mild cramping during implantation, but many feel nothing at all.
Is the baby's sex determined already?
Yes. The sex is determined at the moment of fertilization by the sperm. If the sperm carries an X chromosome, the baby will be female (XX). If it carries a Y chromosome, the baby will be male (XY).
Frequently Asked Questions
What happens immediately after fertilization?
Within hours of fertilization, the zygote begins dividing. By day 3 it is a 16-cell morula; by day 5–6 it has become a blastocyst with about 100 cells. It travels down the fallopian tube and reaches the uterus, where it will implant into the lining.
When does implantation happen?
Implantation typically occurs 6 to 12 days after ovulation, most commonly around day 9. Some people notice light spotting or mild cramping during implantation, but many experience no symptoms at all.
Is the baby's sex already decided at week 3?
Yes. The sex is determined the moment a sperm fertilizes the egg. A sperm carrying an X chromosome produces a girl (XX); a sperm carrying a Y chromosome produces a boy (XY). However, sex cannot be detected on ultrasound until around weeks 16 to 20.
Can I take a pregnancy test in week 3?
Not reliably. The hormone hCG, which tests detect, does not reach measurable levels until about a week after implantation. Testing too early will likely give a false negative. Wait until your period is due (around week 4) for the most accurate result.
Related Articles
- Pregnancy Week-by-Week Overview — see the full timeline
- Pregnancy Week 4: The Positive Test — what comes next
- Fetal Development Trimester Guide, how your baby grows through all three trimesters
Sources
- ACOG - How Pregnancy Happens
- NHS - Week 1 to 3 of Pregnancy
- Mayo Clinic - Fetal Development First Trimester
🔧 Helpful Tools
- Due Date Calculator — Calculate your estimated due date
- Contraction Timer — Track your contractions during labor
- All Baby Tools — Browse all free tools for pregnancy and baby care
