From: Rabbi Akiva Eiger
To: Unknown recipient
Rabbi Akiva Eiger responds to his son's analysis of the Rambam's ruling on betrothing an unborn daughter. He partially affirms the parallel to chatzi shifchah but disagrees that kiddushin takes effect during pregnancy, arguing instead that the word 'if' functions as a temporal condition meaning 'when she is born,' supported by Rambam's Hilchos Ishus 3:11 and the Ran. He concludes with a tzarich iyun on the Nimukei Yosef's reasoning.
Source: Shu"t Rabbi Akiva Eiger §171