The Second Trimester
The fetus increases in size during this trimester, and bony parts of the skeleton begin to form. Fetal movements can be felt by the mother.
The Last Trimester
During this trimester the fetus increases in size. Circulatory and respiratory systems mature in preparation for air breathing. Fetal growth during this time uses large parts of its mother's protein and calcium intake. Maternal antibodies pass to the fetus during the last month, conferring temporary immunity.