Sayang, at 39 weeks, any persistent back pain + tightening perut memang kena check dengan doctor atau midwife ASAP, jangan tunggu lagi, sebab kat stage ni kena confirm if it's real labour or not. A few of my mum friends went through the same, rasa sakit pinggang je, contraction pattern confusing, and dorang kena go to hospital to get checked properly. Only the doctor can tell if it's the real deal or Braxton Hicks, and at your gestation, better safe than sorry! Go get checked sekarang juga, ya?