Three Weeks Old

Today, it's been three weeks since fertilization.
The most surprising thing I learned is that our baby has a yolk sac!

Just like a chicken.  It serves the same purpose, to nourish the growing embryo before the umbilical cord develops.

This is what things should have looked like about a week ago:

The entire embryo is still covered by the uterine lining.

As you can see, the embryo has expanded and now contains a yolk sac and an amniotic sac, with placental cells growing around it.

The cells that will become the baby (here, titled the "embryo," so as not to confuse, ha!) line themselves up into two layers.

Here, it's about the size of a poppy seed.

Here's what things look like now:

The entire thing is still covered by the uterine lining, but boy, those placental cells really have been growing! The third week after conception marks the beginning of the embryonic period, when the brain, spinal cord and heart begin to develop.  Here, where the cells that have lined themselves up in two layers in the previous week, are now becoming the neural tube.  From the neural tube, the brain, spinal cord, nerves, and backbone will develop.

Here's a closer look of the baby:

Isn't it cute?

The baby has three layers: the ectoderm, mesoderm, and and endoderm. The ectoderm (the outer layer of cells) will turn into skin, nervous system, eyes, and ears.  The mesoderm (the middle layer of cells) will turn into the heart, bones, muscles, kidneys, and reproductive system.  The endoderm will become a tube-like structure and the lungs, intestines, and bladder will develop from it.

(what is confusing about finding pictures is that most are considered "5 weeks" at this stage, since the pregnancy is timed at the beginning of the last period.  We know our date of conception, and know our baby is exactly three weeks old.)