Human development from embryo to newborn occurs in three stages: the first, second, and third trimesters. In the first trimester, the embryo develops organs and body parts from two cell layers and fingerprints form. In the second trimester, the skeleton hardens and skin smoothes out. In the third trimester, organs continue developing and the baby gains weight and length. Comparative embryology shows that while human development is similar to other animals in its cell division and layer formation, it differs in the specific organs developed and whether development occurs inside or outside the mother's body, depending on the species.

1. Human
Development
Embryo to Newborn

2. Human Development
GSE: Humans are similar to many species
in many ways, yet unique among Earth’s
life forms.
Let us examine the major changes that
occur in human development from a single
cell to birth

3. First Trimester
(months 1 - 3)
1 month: Your baby is an embryo consisting of
two layers of cells from which all her organs and
body parts will develop.
2 months: Your baby is now about the size of a
kidney bean and is constantly moving. He has
distinct, slightly webbed fingers.
3 months: By now your baby is about 3 inches
long and weighs nearly an ounce. Her tiny,
unique fingerprints are now in place

4. 2nd Trimester
(months 4 - 6)
Month 4: Your baby is now about 5 inches long
and weighs 5 ounces. His skeleton is starting to
harden from rubbery cartilage to bone.
Month 5: Eyebrows and eyelids are now in place.
Your baby would now be more than 10 inches long
if you stretched out her legs.
Month 6: Your baby weighs about a pound and a
half. His wrinkled skin is starting to smooth out
as he puts on baby fat.

5. 3rd Trimester
(months 7 - 9)
Month 7: By now, your baby weighs about 3
pounds and is more than 15 inches long. She can
open and close her eyes and follow a light.
Month 8: Your baby now weighs about 4 3/4
pounds. His layers of fat are filling him out,
making him rounder, and his lungs are well
developed.
Month 9: The average baby is more than 19
inches long and weighs nearly 7 pounds now, but
babies vary widely in size at this stage

6. Comparative Embryology
Let’s compare and contrast human
development to the development of other
animals
The earliest phase of embryonic
development is when the fertilized egg
undergoes rapid cell division
After early cell division, the embryo
separates into three layers, which will
become all of the parts of the animal's
body.

7. Comparative Embryology
Compare and Contrast human development to the
development of other animals
After the layers have completely formed, the
embryo starts developing organs. Often organs
will grow and then disappear, such as the tail of a
human embryo. Also, animals will differ in terms
of what organs they will form because not all
animals have the same structures. Some, for
instance, will develop wings.
The beginnings of embryonic development may
occur inside the mother's body or in eggs which
she has laid outside her body to be fertilized,
depending on the species. Embryonic development
ends when the organism transforms into a fetus
or larvae, after the basic organs are formed.
All embryonic development is controlled by genes
inherited from the parents.

8. Comparative Embyology