I am taking these things figuratively, for the women represent two covenants. One covenant is from Mount Sinai and bears children who are to be slaves: This is Hagar. (NIV)
Now all this is an allegory; these [two women] represent two covenants. One covenant originated from Mount Sinai [where the Law was given] and bears [children destined] for slavery; this is Hagar. (AMP)
Which things are an allegory: for these are the two covenants; the one from the mount Sinai, which gendereth to bondage, which is Agar. (KJV)