​Baby is only a single cell zygote at pregnancy week 3.

Although fertilisation may have taken place, your baby is technically only a single cell big and is called a zygote at pregnancy week 3. It will continue to divide and grow and will be generally known as an embryo until the 10th week of pregnancy.

At pregnancy week 3, the embryo will remain in your fallopian tubes for 3-4 days, multiplying. At the same time, the embryo will move along your tubes in the direction of your uterus. This migration takes 3-7 days.

Boy or girl? Baby’s sex depends on winning sperm’s chromosome

The sex of your child will be determined by the combination of both your and your partner's genetic blueprint. Specifically, it depends on whether the "winning" sperm carried an X or Y chromosome. Mother's chromosome always gives an X. Therefore you would require an XX to make a girl and XY for a boy.

Ref: M19