The basics of making babies seem simple. You need sperm, an egg, and a womb to incubate in. Combine those, and a baby starts to grow. However, most people don't know what develops first in the womb as far as the baby's growth. Does it start with a brain? A heart? What organs come first during pregnancy, and what organs develop last?
The order of organ development in fetuses isn't always exactly the same. Humans all start from the same general blueprint, but some organs start to form around the same time, and some grow faster than others. But here's what science says: babies start as an anus (really), and end by developing lungs. When - and how - body parts grow in between is a long and fascinating process.
Believe it or not, all human beings start out as tiny anuses. In the first few weeks after fertilization, you're nothing more than a small group of cells, called a blastula. This blastula bursts open from the inside out, making a little bitty opening. This opening is called a blastopore, and it is the first of your proto-organs to begin forming. While that may sound like a pretty fancy word, the blastopore is actually just a miniscule anus. The rest of your body develops from there.
As your cells continue to grow and change in the first weeks of development, some of the next things to appear are nerve cells. These nerve cells will eventually help create your nervous system and your sense of touch. These nerve cells will also start forming the first of your vital organs.
Brain, Spine, And Placenta
As an actual body grows from that cluster of cells you started from, a shape will begin to appear. First there will be several layers to the cells, of varying density and formation. Then, a form, like a little dot and a line extending from it, will develop around the fifth or sixth week. These formations are the beginnings of your brain and spine. Though rudimentary, these will help tell your body how to react to their own growth.
The placenta also develops around this time, to help you process nutrients and continue to grow.
Cells will continue to be told by DNA what they're supposed to be doing, and they will continue to change into new functioning organs. Within six weeks, you will begin to develop some odd looking tissue towards the center of your coiled form, in the form of a little s-shaped tube. That tissue will begin to move together and pump like a tiny motor, making a heartbeat at around six weeks.