She saw some drops of blood during pregnancy – should she stop praying?

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.