During pregnancy, there is a major change that occurs to the posture of the mother as a result of her increased body weight. It is this alteration in posture that may be a cause of her headache.
The spine does not remain static during the 9 months. As the uterus grows the lumber spine starts to flatten followed by extending and then flattening again slightly. The ribs too are expected to flare out as the uterus pushes up onto the abdominal organs causing some crowding. These changes that occur result in the neck having to adjust, accommodating in such as way so as to keep the head up straight so that our sensory organs can function efficiently