My gosh - what a horridly condescending comment to make! She was speaking to you as though you were a child. How did you respond? Comments like that can catch us so off guard that we are just stunned in the moment and then regret not saying something later on.
Don't be angry at yourself for that - she was WAY out of line there!

Besides, who's to say that your experience would be the same with a second child anyway? In most cases they are vastly different.

On another note - have you considered NFP (Natural Family Planning)? It is far more comprehensive than the billings method and does not have all the nasty side effects of the pill - one of which can be depression. The hormones in the pill change the Ph level of the body (turning it from a more alkaline environment to an acidic one) and this can lead to brain chemistry changing to conditions that are favourable for depression to develop. No - your doctor will not tell you that, but a Naturopath will.
If you do wish to remain on the pill - be sure to take a high potency probiotic (good bacteria) supplement (ask at your local health food store - needs to be kept in fridge). This will help to counteract the acidic environment that the pill creates and prevent you from getting subsequent acidity-based conditions such as thrush, candida, digestive problems, headaches and urinary tract infections.

All the best in dealing with your MIL. Sometimes silence is golden - and other times they need to be put in their place!

XX