Mishnayos Gittin Perek 8 Mishnah 4

בגט ישן – He [i.e., the husband] wrote to divorce his wife and after the Jewish bill of divorce had been written, he was [found to be] alone with her. The School of Shammai holds that we do not say that there is a decree lest people would say that her Jewish bill of divorce came before her children [arrived] lest the [delivery of the Jewish bill of divorce] is delayed a year or two [years] between the writing and the delivery, and she will have had children from him in the midst of that time and afterwards, will divorce her with it; and when people will see the date of the Jewish bill of divorce which is prior to the birth of her son, people would think that he [i.e., the husband] gave her the Jewish bill of divorce at the time it was written, and there would be a discredit [to her] as [people] would say that the children were born from a “free” (i.e., unmarried) woman, and the Halakhic decision is that a person should not divorce his wife with an “old Jewish bill of divorce.” But if the husband divorced her and then went to another country, she may ab initio, remarry through this.