1) The Midrash Rabah to Esther 4:4 (Parshah 8:3 in the Midrash) cites Rebbi Yudan b'Rebbi Shimon who says that Esther used a "Moch." However, immediately following this, the Midrash says in the name of Rebbi Yudan that the last Darius was the son of Esther. The Matnos Kehunah explains that once Achashverosh promised Esther that her son who will be born from him will build the Beis ha'Mikdash (as stated in the Targum), Esther no longer uses a Moch.

2) The above Targum is on Esther 7:2 (at the end). Achashverosh said to Esther, "Wait until your son Darius will grow up and inherit the kingship, and he will build the Beis ha'Mikdash."

3) The answer of the Maharsha:

Rabah bar Leima says (Megilah 13b) that Esther would arise from being with Mordechai, immerse in the Mikveh, and then be with Achashverosh. Tosfos (DH v'Toveles) asks how could she have done this? There is a requirement to wait three months between leaving one husband and marrying another in order to discern the identity of a child that might be born. Tosfos answers that she used a Moch.

It appears from Tosfos that the reason she used a Moch was not so much so that she should not become pregnant, but rather so that she not give birth to a child whose father would not be known. It seems to me that this is the reason why the Maharsha (in Chidushei Halachos to 13b; notice that he does not write this in his Chidushim on the Agadah), writes that she used the Moch only for Tashmish with Achashvevrosh. When she was with Mordechai she did not use a Moch. This is because only three women have a Heter to use a Moch (a girl under 12, a pregnant woman, and a nursing mother; see Kesuvos 39a). Esther did not fall into any of these categories. Tosfos in Kesubos (39a, DH Shalosh) writes that the reason why other women are not allowed to use a Moch is that it wastes the seed of the husband. If so, this prohibition would not apply to Achashverosh, which is why she was allowed to use a Moch with him.

The Maharsha writes that after she became prohibited to Mordechai, as the Gemara states below (15a -- "Ka'asher Avadeti... Kach Ovad Mimecha," and see Rashi DH Avadeti Mimecha), she stopped using the Moch and then became pregnant. According to the Maharsha, she was careful to use a Moch when she was also with Mordechai, so that she would know who the father was, but when she was no longer with Mordechai there was no need for a Moch.

4) The Turei Even says the opposite of the Maharsha:

The Turei Even (Megilah 13b, DH v'Toveles) answers this question by writing that Esther was afraid to use the Moch for Tashmish with the Rasha, Achashverosh. It was only for Tashmish with Mordechai that she used the Moch for Havchanah, to make sure that it would be clear who would be the father of her child. This is how she became pregnant from Achashverosh, with Daryavesh (Darius).

This disagrees with the Maharsha who writes that it was forbidden for her to use the Moch with Mordechai since she was not in one of the three categories of women which Kesuvos 39a states are permitted to use a Moch.

Kol Tuv,

Dovid Bloom