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It is the sperm decides it and there are a lot of thoughts on what causes a certain sperm to get there first and fertilize an egg. Some say male sperm are slow and female are faster so have sex before you ovulate to get a girl and later to get a boy. This is not 100% either. It is just God, Fate, luck of the draw, whatever you want to call it
It is definitely determined by the sperm that reaches the egg. There may be some truth regarding pH levels and how that affects the sperm, and when you have sex in relation to when you ovulate, but it's mainly speculation.
The PH of the mother only pertains to the ease of the sperms reaching the egg becuase Y sperms and X sperms like boys and girls are completely different about how they go about things. There is no solid evidence to back up PH levels and sex determinations though. But in some of the college classes I had to take for kinesiology talk about it to a start but it simply has no solid background. But ultimately PH has nothing to do with determining sex of baby. Simply X or Y