First trimester · 3 weeks pregnant
Week 3: fertilisation and the journey to the uterus
A single cell becomes a ball of cells and starts to implant.
The fertilised egg divides again and again as it travels down the fallopian tube. Around the end of this week it reaches the uterus and begins to burrow into the lining.
Size and weight
About the size of
a grain of fine sand
Length
0.1 mm
Weight
—
Averages from standard growth charts. Measured crown to rump until week 20 and head to heel afterwards.
How your baby is developing
- The zygote divides into a cluster of cells (the blastocyst) within about five days.
- The outer cells will become the placenta; the inner cells will become the baby.
- Implantation into the uterine lining starts six to ten days after ovulation.
- Sex and most genetic traits were decided at fertilisation.
How you may feel
- Usually nothing at all
- Light implantation spotting or mild cramps in some women
- Early breast tenderness from rising progesterone
The pregnancy hormone hCG only starts to rise after implantation, so a test is still too early this week.
What's next
Tests and appointments
No routine tests this week. Keep taking your vitamins and contact your polyclinic with any concerns.
What to do this week
- Keep taking folic acid; the neural tube forms in the next three weeks.
- Treat yourself as if you might be pregnant: no alcohol, and check medicines before taking them.
- Try not to over-analyse symptoms. Progesterone causes the same feelings whether or not conception happened.
Call your doctor or maternity hospital if
- Bleeding that is heavier than light spotting, or bleeding with cramps
- Severe pain on one side of your lower abdomen, or pain with dizziness or fainting
- Vomiting so often that you cannot keep fluids down for 24 hours
- A temperature of 38 °C or higher, or pain when you pass urine
If something feels wrong, trust yourself and call 103 or your maternity hospital. You will never be wasting anyone's time.
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