Though this new practice is fantastic for Mother's and Father's. It's leaving employers, particularly small employers in quite the bind. Temporary employees used to replace new Mothers/Fathers tend not to perform or provide the same output as permanent employees. 
Rights dictate that employers are not allowed to discriminate against women but with women having children in their 20's and 30's even their 40's it's becoming difficult for employers to justify hiring employees that will be out of the work force for 18 month stretches. Many families expect to have two children, making this a three year absence from work. 
A thinly veiled question is usually asked in interviews, "do you have children?" Though it appears on the surface that the potential employer is being friendly. This type of question when posed to women in their 20's and 30's is really a queue for them to eliminate these currently childless women from the running for that position.