Keyword Birth Control Pill

In this 1970s comedy, a Catholic man contemplates going against his priest's guidance on contraceptives when his wife wants to stop having children.

Prudence Hardcastle is on the pill. So is her sister-in-law, but someone has been swapping aspirin for their pills. Is it the teenage niece, the maid, the chauffeur, a lover, Prudence's husband Gerald, or all of the above?