Is spotting normal in first trimester for twin pregnancy?

Hi mummies, I'm carrying twins but had experienced spotting for 3 occasions (5 weeks, 9 weeks, 11 weeks) already. First 2 times was pinkish red and this time was brownish. The spotting usually comes only when i wipe after using the toilet, and goes away after that. The 1st time i was asked to up my medication dosage, 2nd time I had a scan and babies were growing well. I had another scan few days back to confirm everything is ok. Gynae mentioned babies are all good. But I just had spotting again (Week 11). Read that placenta is growing rapidly this week, does this cause spotting?