Membrane sweep

Im 38 weeks and 1 day. Baby has been consistently measuring over 90percentile despite my diet controlled gestational diabetes. Abdominal circumference is always 98% which suggests large shoulders are more likely.

So today i went in for a routine check up and ob asked if i wanted to have my cervix checked and i said sure! She looked and said i was 1.5 dilated and 100% effaced which surprised her cuz i havent had any contractions that i know of. She scheduled me for a membrane sweep next wed.

I was just wondering how effective it is and whether dilation or thinning affects its effectiveness?