It is haram for a Muslim woman to continue fasting when she is menstruating. If she gets her period while fasting, her fast is spoiled, even if the blood comes a moment before sunset, and she has to make that up if it was an obligatory fast.

If a woman had an abortion before 80 days of pregnancy, then the blood she passes is not the blood of Nifas which does not stop her from praying and fasting, but she has to do Wudu for each prayer. If the abortion took place after the soul was breathed into the foetus, then the blood is definitely Nifas and she cannot pray or fast.

The blood that applies in the case mentioned in the question is irregular vaginal bleeding. These drops of blood are not menstruation and do not mean that a pregnant woman should stop praying and fasting.