From: Rabbi Moshe Sofer
To: Rabbi Yosef Shmuel Segal
The Chasam Sofer rules that where labor contractions ceased and midwives confirmed the cervix was sealed, the matter is retroactively clarified that the womb never opened and the wife is permitted to her husband. Regarding a woman with a twice-monthly cycle who has not yet fulfilled pru urvu, he permits counting shivah nekiim after three onos rather than six, and allows relations on the night before the daytime veses onah, on the grounds that the relevant stringencies are lo plug rabbinic extensions that may be relaxed to prevent divorce and enable procreation.
Source: Shu"t Chasam Sofer YD §179