Many of the scholars are of the view that a pregnant woman cannot menstruate. This is the view of two imams: Abu Haneefah and Ahmad (may Allaah have mercy on them).
See: al-Mughni, 1/443.
This view was also favoured by the scholars of the Standing Committee for Issuing Fatwas.
Others are of the view that a pregnant woman may menstruate. This is the view of Imam Maalik and Imam al-Shaafa’i (may Allaah have mercy on them).
See: al-Majmoo’ (2/411-414)
This view was favoured by Shaykh Muhammad ibn Ibraaheem and Ibn ‘Uthaymeen (may Allaah have mercy on them).
This is so long as the blood meets the description of menstrual blood and comes at the normal time of menses.
This has been explained in the answer to question no. 23400.
Whatever the case, these drops that have come out from your wife are not menses, because they do not meet the description of menstrual blood and are not occurring at the time of menses.
So your wife is pure (taahir) and she may pray, fast and do all that women who are taahir do.