Hi momsh! Honestly, this is such a legal-heavy question na kailangan mo talaga ng clarity from the Civil Registry or maybe a lawyer, kasi ang surname rules eh strict talaga at may specific requirements depending on your situation. A few of us moms have gone through paternity stuff, and yung consensus is na you really need to talk to the civil registrar mismo or a legal aid office para alam mo exactly what docs you need and kung pwede talaga gawin solo, Hindi ka naman kailangan ng ex's presence for affidavit of paternity, but the process ay may specific steps talaga. As for the paternal grandmother wanting to use their surname, I'm not sure kung may say siya diyan, pero ang civil registrar talaga ang makakacompute kung ano ang valid options for your baby's name. Congrats on your first baby, get that advice settled before you deliver para walang stress, okay?