Feeling very tired is normal in pregnancy, starting early on.
A woman can start feeling unusually fatigued as soon as one week after conceiving.
Why? It's often related to a high level of a hormone called progesterone, although other things -- such as lower levels of blood sugar, lower blood pressure, and a boost in bloodproduction -- can all contribute.
If fatigue is related to pregnancy, it's important to get plenty of rest. Eating foods that are rich in protein and iron can help offset it.