Yes, it's normal. Most women begin to wear sanitary pads at this time.
I had this same question when I was 20 weeks and it was just "wet" down there, no trickling for me, just a lot of moisture down there and my OB told me if it was amniotic fluid it would be trickling down my legs quite frequently. I would definitely contact your midwife and let her know, they have a test strip that they just wipe you with and it will turn a color if it's amniotic fluid, better safe then sorry! Good luck