Melenggang Perut ? for 1st Child

Assalamualaikum mummies. Saya ada kemusykilan ni. Mak mertua saya minta kami melenggang perut nanti bila usia kandungan dah 7/8 bulan. The problem is, yg sy tahu amalan ni adat lama melayu and Islam dah HARAM kan. And sy tak prefer nak jalan jauh (balik kelantan) for perkara yg sy tolak ni. And husband pon macam berbelah bagi. Sebab im the only 'menantu luar' jadi sy taknak nampak yg sy ni rude untum tolak mentah2. So, in your opinion how should i tell them about this? Now im googling about this to get the exact answer ?