Kebas kat punggung dan kaki masa hamil tu common, sis, biasanya sebab berat badan yang bertambah dan perubahan postur, tapi memang la susah nak duduk dengan selesa! Since you're feeling it quite a bit at 18 weeks though, better check with your doctor just to be sure everything's okay, especially if the numbness is getting worse or affecting your daily life. A few of us mum friends yang pernah rasa macam ni, derang try bantal sokongan kat belakang bila duduk, atau stretch skit-skit untuk hilangkan ketegangan, paracetamol pun boleh tolong if it's discomfort rather than serious pain. Jangan tahan je, go see your gynae soon ya, baru kita tau if ada something else yang perlu dicheck!