If you are sure of pregnancy after having tested for it even though you have been on birth control pills, The most likely cause of contraceptive failure is usually irregular pill intake or missing pills regularly which cause losing of protection.
Regarding your query, yes the early pregnancy can abort with withdrawal bleed in the early months but not necessarily the pill may be the cause , but yes other hormonal imbalances and immature endometrium can cause deficient implantation and thereby abortion.
You may undergo an ultrasound and a per speculum exam and if required a D&C to complete abortion and send products for histopathological exam to confirm the pregnancy .
I hope i have answered your query,
These Q&A’s are for educational purposes and should not be relied upon as a substitute for medical advice you may receive from your physician. If you have a medical emergency, please call 911. These answers do not constitute or initiate a patient/doctor relationship.