The answer is very precise and is a simple yes, it is possible and advisable to take the precautions necessary, nay, the abdominal muscles work hard throughout the period and especially in the time of delivery, so you should either be medium toned and trained by then. The only risk (provided you are enrolled in a normal pregnancy without contra indications of any kind), is the possibility of decreased blood pressure when lying face up.
The recommended exercise is a variation on the abdominal floor: looking to the floor, his hands and knees on him, arching his back up and then using the abdominal muscles relax and never abruptly. Now, the classic abdominal exercises upside down can perform no more than the third month after that is dangerous to the fetus and difficult for you.
Another proposal: standing upright with your back, knees without lfexionar and abs, slowly shrink the perineal muscles (near vagina) for 8 seconds, then relax suavemente.En general, any exercise you do (walking, lifting arms etc) by contracting the abdominal muscles gently but thoroughly, will be a way to tone and oxygenate the area and your whole body, ie all profits for you and your baby.