An overview

Week 2 Conception is the moment at which the sperm penetrates the ovum. Once fertilized it is called a zygote, until it reaches the uterus 3-4 days later.

 

Week 4 The embryo may float freely in the uterus for about 48 hours before implanting. Upon implantation, complex connections between the mother and embryo develop to form the placenta.

 

Week 6 The embryo is about 1/5 of an inch in length. A primitive heart is beating. Head, mouth, liver, and intestines begin to take shape.

 

Week 10 The embryo is now about 1 inch in length. Facial features, limbs, hands, feet, fingers, and toes become apparent. The nervous system is responsive and many of the internal organs begin to function.

 

Read More on the First Trimester and view stunning photos of fetal development!

Examining hands and feet
Arms and hands develop earlier than legs and feet, but by two months, the lines of palms and footprints are already engraved in the skin.

* Obstetricians count “weeks of pregnancy” from the first day of a woman’s last menstrual cycle because there is often no way to determine exactly when conception occurred. Embryologists, however, typically describe the developing embryo or fetus by the number of weeks since conception. On these pages we count weeks of pregnancy from the first day of the last menstrual cycle. To determine the age of the unborn child since conception using this table, subtract two weeks.
Introduction to Child Development, 6th ed., John Dworetzky,
West Publishing Company, San Francisco, CA, 1996, p. 70-75.