Beginning with conception, this is a detailed daily, weekly and monthly general overview of fetal development by The American Pregnancy Association. Depending on the mother's health, development may vary among pregnancies.
Overview: Fetal Development
Reprinted with permission from American Pregnancy Association The following is a daily, weekly, and monthly description of what is happening in the uterus and the developing fetus from conception, which is two weeks after the last menstrual period. (The actual pregnancy began two weeks ago, on the first day of the woman's period. During the two weeks before conception the reproductive system was preparing for a pregnancy.) This is to be used as a general guide for healthy development. Development may vary among pregnancies due to mother's health. For more details and resources, check out Pregnancy.org's Pregnancy Calendar.
Day 1: Sperm joins with the ovum to form one cell smaller than a grain of salt. Twenty-three chromosomes from each parent join to form every detail of human development: sex, hair, eye color, height, skin tone, personality, emotional make-up, and other inherited characteristics.
Day 3-4: The fertilized egg is rapidly dividing as it travels down the fallopian tube into the uterus where for the last two weeks the lining has been preparing to receive the zygote.