Well I don't think you did anything wrong by standing your ground. As otherse have said I also fail to see what the advantage would be of getting a window seat when being pregnant. To the contrary, for quick bathroom access one would think the aisle seat would be preferred. To me the 'she is pregnant' argument was only used because the pax wanted a window seat - pregnant or not.

Once on SQ in C I had ordered a 'Book the Cook' meal (think Lobster Thermidor) and a pregnant woman was insisting that she wanted that meal choice. When the FA explained that this was a pre-ordered meal and that there was only one for me she told her to ask me to give it up because she is pregnant. Needless to say that she had to pick from the menu out of 3 very decent main courses as I had already stuck my fork into it. Sometimes one has to put an end to the entitlement thinking even if it means that a pregnant woman can't get her ways for once in a 9 months period