4 weeks

Not sure, but this is where I think I am:

Pregnancy Weeks 1 to 4

The Baby:
The fertilized egg is undergoing rapid cell division. It will implant in your uterine lining this week. Once implantation occurs, it's called a blastocyst and is very tiny. It only measures 0.1-0.2 mm in diameter.

A yolk sak has developed and will be the nourishment for baby until the placenta is formed and takes over. Did you know that the sex and other characteristics of your baby have already been decided? It's all in the DNA code that was formed when the sperm and egg first met.

Another version:
WHAT'S HAPPENING WITH YOUR BABY

The fertilized egg has made its seven- to ten-day trip to the uterus, where it implants cozily into the endometrial surface. It is now called a blastocyst and it has divided into two parts. While you are reading this, the first part is forming the placenta and the second part will become the embryo. The two layers that make up the amniotic bag of waters are newly formed, and the very beginning of the connecting stalk -- which will become the umbilical cord -- has appeared. Until your placenta is fully functioning, the yolk sac, now present, will feed your baby. Basically, the "home" for your little one is under rapid construction!