Officially, you must be able to lift a certain specified weight (35-60 lbs, I believe) without difficulties, and also be able and willing to assist the crew in an emergency exit, including (but not limited to) assisting other passengers to exit the plane.

For someone who is in their first trimester, or early second, this probably wouldn't be an issues except in some guarded cases. Much beyond mid second trimester would be stretching it, IMO. For that matter, I would also raise an issue for someone wearing a neck brace, an arm in a sling, an elderly person with frail composure, an obese person who become winded just walking down the aisle, or anyone using crutches. Just common sense, really, and in no way is discriminatory about the personal capacities of an individual person. Suffice it to say that if my burcitis (sp?) was really acting up for a particular segment I would notify an attendant to excuse myself from the row.

GMF