Final answer:
IgG is the only class of immunoglobulins that crosses the placenta, providing protection to the fetus during pregnancy.
Step-by-step explanation:
The only class of immunoglobulins that crosses the placenta is IgG. IgG is a major antibody in the blood and plays a crucial role in providing passive immunity to the developing fetus during pregnancy. It is the most abundant antibody in human blood and can efficiently penetrate tissue spaces, protecting the fetus from diseases.