Week 3 of Pregnancy

Baby Size Comparison in Week 3

In week 3 of pregnancy your baby measures about Under 1 mm long and weighs about Under 1 gram: roughly the size of Poppy seed. Remember these are approximate averages from standard growth charts; every baby grows at their own pace, and your doctor follows your baby's personal growth curve across visits, not a single number. If an ultrasound measures a little bigger or smaller than average, that is usually normal as long as growth continues at a steady rate.

Baby Development in Week 3

Fertilization has happened! In the fallopian tube, a single sperm has penetrated the egg's wall and the two sets of genetic material have united to form the fertilized egg (the zygote), which carries the child's complete genetic traits; in this very moment the baby's sex is determined: if the sperm carried a Y chromosome it is a boy, and if it carried an X it is a girl.

The fertilized egg immediately begins dividing rapidly as it rolls slowly toward the uterus: two cells, then four, then eight, until it becomes a ball of cells called the morula, then the blastocyst. The journey through the tube takes about three to four days, and throughout this period the dividing cell feeds on its internal reserves before reaching the uterus and seeking a place to implant.

After fertilization, a protective layer called the zona pellucida forms around the fertilized egg; it prevents other sperm from entering and stops it from attaching too early to the tube wall. The cells divide nonstop, and around day five the hollow ball called the blastocyst is ready to implant. Identical twins arise when this ball splits into two parts, while fraternal twins arise when two separate eggs are fertilized. During the journey to the uterus, the cells depend on their internal food reserves before they start feeding from the uterine lining after implantation.

Your Body Changes in Week 3

In week three you usually feel no symptoms at all; your body has not detected the pregnancy yet because the pregnancy hormone has not risen. Some women notice, at the end of the week, very light spotting called implantation bleeding, with a mild twinge in the lower abdomen, but most women cannot tell anything.

Progesterone gradually starts rising to support the uterine lining and may cause mild sleepiness or breast tenderness resembling pre-period symptoms. A home pregnancy test this week is too early and will most likely come back negative even if pregnancy has occurred; wait for your missed period.

Progesterone supports the uterine lining through the corpus luteum, which formed where the follicle ruptured, and it will keep up this support until the placenta takes over the job. You may feel mild drowsiness and breast tenderness. Avoid excess stress and anxiety; mental calm supports hormonal balance. And carry on with your life normally without drastic changes.

Practical Tips for Week 3

• Avoid alcohol, smoking, and any medication without medical advice starting now; the organs will begin forming soon.
• Do not take a pregnancy test before your missed period; an early result is often a false negative and causes frustration.
• Keep taking folic acid daily; the neural tube will begin forming within days.
• Avoid very hot baths and saunas; severely raising body temperature is unsafe in early pregnancy.

Frequently Asked Questions About Week 3