From: Rabbi Moshe Sofer
To: Rabbi Chaim
When a woman miscarried on Yom Kippur and the fetus was not examined, the obligation of pidyon haben for the subsequent son depends on whether the mother's pregnancy was established. If she was huchzekah ubrah, the Kohen has no monetary claim; if uncertain, the father should give the redemption money conditionally (al menas lehachzir) with no bracha. Where women did examine the shafir and found it unformed, the Chasam Sofer follows the Maharik and Rema requiring pidyon, with the bracha recited without Shem u'Malchus to accommodate the Chacham Tzvi's dissent.
Source: Shu"t Chasam Sofer YD §299