Is abnormality scans vital around 20 weeks?I'm 26weks & haven't had it yet can it wait?

My partner is Visiting his sick grandma abroad & he comes back in 5 weeks-I'll be 8 months then; that too late to have my 2nd scan to check for abnormalities?he really wants to be there but if it's vital to have it now I will have to go without him :( any advice? All is helpful thankyou x