14 weeks is plenty. Please don't overdo it. What I hate to see is people thinking "well, if 12 weeks is good, 17 is better" and then dieting strictly for 17 weeks (to use your example there LOL) and then starting off having wasted away to nothing already, then starting off with a very strict sway that has no chance of conceiving. Then after 2 BFN, they either have stopped ovulation and/or are in such a panic to conceive that they immediately stop all sway tactics and start swaying blue just in order to get pregnant. This one mistake causes more sways to fail than anything else.