I do agree with the statement that first trimester for pregnancy is the most important.
The most dramatic changes and development occur during the first trimester. During the first eight weeks, a fetus is called an embryo. The embryo develops rapidly and by the end of the first trimester it becomes a fetus that is fully formed, weighing approximately 1/2 to one ounce and measuring, on average, three to four inches in length.
With so many development taking place, no doubt there is a sudden change in hormone. That should be the reason most mummies would feel tired or even nausea with the sudden change in the body.
Time to sleep and eat, mummies....You have got all the reason for that :)