Do you want to know what I think? Here is my theory on those who end up with BBBG or GGGB without swaying.
The moms who naturally make boys are so frazzled and stressed out after having their third boy because they are outnumbered and can't control things like they could with 1 or 2 that they have declining conditions. Hence, they don't have time for stress and worry and not as much time for regular sex.
Then the moms like me who naturally make girls and always tried to stay thin and healthy (cardio) just give up with 3 girls and do whatever to get by. For example they eat more than they should when they are finishing the kids food or just snacking while making dinner, and don't bother exercising because they can't find time anyway, or those who are adamant exercisers throw themselves into exercise with the shortest and fastest workout like Crossfit, and other quickies that produce good results.
I recall clearly hearing MIL say "After DD2 (her DC3), I gave up trying to lose weight and stay slim". She went on to have 4 boys after that.
And Shettles theory is nonsense, we all know that.
