What is the cause of swollen feet in 39 weeks pregnancy?

You'll find out in: The next two weeks :-). The baby and all the rest that you're carrying in the front puts some pressure on the veins coming from your legs into the pelvis. This causes a back pressure that results in the water retention found so often in pregnant women. Elevate your feet as often as possible, and be patient. Best of luck with the new baby.

Like any mass...: The baby is a mass growing in your pelvic region, and it causes pressure on some of the blood vessels obstructing the blood flow from your legs back to your heart, thus causing your legs accumulate fluid. Although it largely improves after the pregnancy is over, it may lead to the formation of the varicose veins and some leg swelling to remain in the future.