Most contemporary books on the laws of niddah state that the husband cannot attend the delivery because his wife is niddah, and he might come to see parts of her body that are normally covered. This opinion is also advocated in articles such as that by Rav Steinberg In his article "Natural Childbirth: May the Husband Attend?" in Journal of Halacha and Contemporary Society 1981;1:107-122. Among responsa, you have the Minchat Yitzchak 8:30:2, in which he indicated that he prohibited this practice when he was asked, even when the husband was not looking. Other poskim maintain that a woman's need for support during childbirth outweighs this concern, and thus the husband may (and perhaps should) attend. Sources who permit the husband's attendance in the delivery room are Rav Moshe Feinstein (Igrot Moshe Yoreh Deah 2:75) and Rav Yehudah Henkin (Bnei Banim Vol 1 Number 33). Rav Yaakov Ariel has also stated in public lectures that this is permissible.