Just because Agnus Castus does not require a prescription and is a ‘natural’ herb available from ‘health’ shops does not mean to say that it cannot have strong effects (although it probably doesn’t). Chemicals called flavonoids have been extracted from the herb, which have been shown to affect the hormone prolactin (the hormone responsible for milk production). Therefore Agnus Castus has been used to try to relieve cyclical breast pain and PMT.
There is one scientific paper reporting that one woman who took Agnus Castus ovulated more eggs than would have been expected, so it may act as a stimulant of ovulation. This has yet to be proven. It seems unlikely that the herb would do any harm, but good advice is always to take as little as possible during pregnancy. If Agnus Castus has any beneficial effect, it will be before ovulation, so perhaps taking it only for the first half of your cycle would be the most sensible.