SSS MAT1 ultrasound requirement

18 weeks pregnant here. Ask ko lang regarding SSS MAT1 requirement - okay lang ba na yung 1st tvs result yung ipasa ko kahit 18 weeks pregnant na ko ngayon? Last May 10 pa kasi yun and around 8 weeks ako non. Or kailangan ba yung latest? Btw, I am employed po kaya sa HR ko ippasa to.