False positive pregnancy tests? Why?

Patient: I went to hospital about 1.5 months ago to get IUD merina and they did urine pregnancy test which came back positive, (so IUD was not put in) they sent me for a blood test which came back negative. I have just gone back to get the IUD put in, another urine pregnancy test done, and it came back ‘boarderline’ positive. I am being sent for blood test in 2 weeks…..has anyone ever had anything similar happen to them? my youngest child is 5.5 months old (no period since he was born), could I still have hormone in my body from that Thank you.

Doctor: You could have a false-positive result if blood or protein is present in your urine. And certain drugs, such as tranquil izers, anti-convulsants, or hypnotics, may also cause false-positive results. If two tests show different results, a blood test is a good idea to confirm results.