What is the source that states when Moshiach is here - women will deliver babies the moment they conceive. I heard this in a shiur from HaRav Yitzchak Ginsburg in Israel.
The source is probably the Talmud (Shabbos 30b):
כי הא דיתיב רבן גמליאל וקא דריש עתידה אשה שתלד בכל יום שנאמר (ירמיהו לא, ח) הרה ויולדת יחדיו
Rabban Gamliel was sitting and he interpreted a verse homiletically: In the future, in the World-to-Come, a woman will give birth every day, as it says: “The woman with child and her that gives birth together” (Jeremiah 31:7), explaining that birth will occur on the same day as conception. (Sefaria)