First trimester fatigue may start as early as one week after conception for some women. The hormone progesterone, in particular, plays a significant role in making you tired due to changes in your hormone levels. During the first trimester, this hormone increases dramatically. Additionally, your heart beats faster and harder as blood volume rises to feed the growing placenta and fetal circulation. It's also possible that low iron levels are making you fatigued, but this is more typical in the latter stages of pregnancy. The severe tiredness of the first trimester is soon forgotten by most women as the glow and increased energy of the second trimester sets in. In the third trimester, fatigue frequently returns as a result of sleep disruption and growing discomforts.