Better ways for treating infertility?  Or better ways to conceive?  In a population as interbred as Pure Blood Wizards, infertility and low fertility might be a logical genetic problem.  Given the number of only children, it might be a widspread problem, to see low fertility.

However, given that Wizard society is so very conservative on so many levels, things like IVF and sperm banks (or the magical equivalent) may just not exist.  Or be prohibitively expensive.

And given how fertility treatments are so very difficult, the tried and true methods are often the surest.  Even in the Muggle world, ovulation can be brought on by medications, so I'm assuming a potion or charm exists to help that.  If Lily had been trying to conceive for a year, and was studying healing, she'd be well-versed in all of it.

And, as Snape has said,  James was the father in every way that mattered.