Most of the human formation occurs in the first trimester. They spend the second and third trimester growing, further developing those systems, and gaining weight to prepare for life outside the womb.
The above list is only an overview of the major milestones. The process of development from conception to birth is a journey of intricate detail. You can read about a baby’s development in detail on the following websites.
The Endowment for Human Development
The Virtual Human Embryo
Visembryo is a scientific website that goes into great detail on every step of their development.