Birthing parents at Amazon are offered up to 20 weeks of maternity leave. bluestork/Shutterstock Parental 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 finances and medical expenses to one's lifestyle and living situation. But in the US, another sometimes crucial detail is the companies where parents work. The US is the only developed, high-income nation that doesn't guarantee national paid parental leave , according to the  BBC , although 14 states and Washington, DC, have enacted their own mandatory paid leave programs. Employers' policies can therefore make all the difference in how family planning and postpartum care look.…