6 Weeks - According to Babycenter:

"This week's major developments: The nose, mouth, and ears that you'll spend so much time kissing in eight months are beginning to take shape. If you could see into your uterus, you'd find an oversize head and dark spots where your baby's eyes and nostrils are starting to form. His emerging ears are marked by small depressions on the sides of the head, and his arms and legs by protruding buds. His heart is beating about 100 to 160 times a minute — almost twice as fast as yours — and blood is beginning to course through his body. His intestines are developing, and the bud of tissue that will give rise to his lungs has appeared. His pituitary gland is forming, as are the rest of his brain, muscles, and bones. Right now, your baby is a quarter of an inch long, about the size of a lentil bean."