Seconds after my daughter was born a nurse placed her tiny pulsing body on my chest. The sight captivated me, and as she lay there I thought of nothing else but the marvel of her new life.

In the days that followed, the complicated and busy world came into view again, and I began to hear the inevitable ticking of time. In 12 weeks, I would somehow have to find the courage to ignore the hormones that primed my body and mind for complete devotion. I would have to leave this helpless being, my first child, and return to work.

I’d taken for granted that 12 weeks of leave is a standard benchmark, thanks to the Family and Medical Leave Act, which requires companies of a certain size to give employees that much unpaid leave in the event of childbirth or adoption. But I couldn’t explain how we arrived at that number. Surely it must be based on the recommendations of child development experts, or reflect the difficulty of caring for a three-month-old baby, or even take into consideration the physical recovery after childbirth.

It turns out that none of these things is true. Indeed, the amount of time American women (and men) are given to care for a newborn is based on nothing but decades-old politics. As the president and Congress look to expand paid parental leave for workers, that number remains incontrovertible despite evidence that longer leave can be better for families.