Birthing parents at Amazon are offered up to 20 weeks of maternity leave.bluestork/ShutterstockParental leave policy, and specifically maternity leave policy, varies heavily across US companies.UCLA researcher Hillary Cookler tracks the parental leave policies of the 500 largest US companies.Eighteen companies stand above the rest in terms of transparency and length of leave.A host of factors go into deciding whether to have a child, from financ…