In the times of the Gemara, there was usually almost three years, at least, between one birth and the next. After a woman gave birth, she would usually breastfeed the baby for 24 months. See Kesuvos 60a that according to Rebbi Eliezer the baby should be nursed only for 24 motnhs, and according to Rebbi Yehoshua even four or five years is acceptable, and the Halachah follows Rebbi Yehoshua. See end of Yevamos 42a that one should not marry a woman nursing a baby from a previous husband. Yevamos 43a states that the Halachah is that she must wait 24 months before remarrying. We learn that mothers frequently breast-fed for 24 months. See Nidah 7a, in the Mishnah, that one of the four women who are "Dayan Sha'atan" is the nursing mother. The reason is because she does not see blood, since a nursing mother does not usually have menstrual bleeding. Since the Gemara in Nidah 31b states that a woman becomes pregnant only near her "Veset" of bleeding, it follows that nursing mothers do not usually become pregnant. Tosfos to Nidah 10b (DH d'Havyah) writes that this means that the majority of nursing mothers do not become pregnant, but sometimes it happens that she does.

It follows that usually there is at least 24 + 9 = 33 months (i.e. almost 3 years) between one baby and the next. The people in Yevamos 119 who went overseas were away for less than that time, so there was not time to have two babies.

Dovid Bloom