The blood which may be passed during pregnancy may be menstrual blood, or it may be irregular vaginal bleeding, or it may be the blood of nifaas. It is the blood of nifaas if it comes at the time of labour contractions. Some of the scholars said (that it is the blood of nifaas) even if it comes two or three days before delivery.
Shaykh al-Islam said: With regard to that (blood) which is seen two or three days before delivery, it is nifaas, because it is blood that comes out because of giving birth, so it is nifaas just like blood that comes out afterwards. This is because pregnant women rarely see blood, but if a pregnant woman sees blood close to the time of delivery, it is obviously because of birth, especially if she is having contractions.
Sharh al-‘Umdah, 1/514, 515
It is menstrual blood if it has the characteristics of menstrual blood and comes at the time when she usually menstruates. This is the view favoured by Shaykh Muhammad ibn Ibraaheem and Ibn ‘Uthaymeen. See Fataawa Muhammad ibn Ibraaheem, 2/97.
Shaykh Ibn ‘Uthaymeen said:
If a pregnant woman experiences persistent bleeding that comes at the time of the month when she usually menstruates and looks like menstrual blood, then it is most likely to be menstruation and she should stop praying, fasting etc, because of that. The only difference with regular menstrual blood is that it has no bearing on the ‘iddah [in case of divorce or widowhood], because pregnancy is the stronger factor in that case.
Al-Sharh al-Mumti’, 1/405
The view that a pregnant woman may menstruate is the view of al-Shaafa’i and was narrated in one report from Ahmad; it was also the view favoured by Shaykh al-Islam Ibn Taymiyah, as stated in al-Ikhtiyaaraat, p. 59; but it is very rare for pregnant women to menstruate.
It is irregular vaginal bleeding if it is something other than that described above (i.e., menstruation or nifaas); this is red blood that is discharged in the case of istihaadah (irregular menstrual flow). This is what some women call al-nazeef (lit. haemorrhage).
This does not mean that a woman should not pray or fast; rather she comes under the same rulings as women who are taahir (pure, i.e., not menstruating).
Fataawa al-Shaykh Ibn ‘Uthaymeen, 2/270
This third category is what applies in the case mentioned in the question, because these drops of blood are not menstruation and do not mean that a woman should stop praying and fasting, and she does not come under the rulings on menstruation.
And Allaah knows best.