All long bones short at 18 weeks pregnant. Increased nt at 12 weeks pregnant. Any chance of a healthy baby?

I am a worried mum to be aged 20 years old. I had an increased nt when I went for my 12 week scan which was measuring 3mm. I was then given the option of having a diagnostic test which I refused and opted for further scanning. I had a further scan at 17weeks 4days and all long bones are measuring short. There was also a mild pericardinal effusion. The sonographer has ruled out Trisomy 21 however still feels this is a significant risk for other chromosone disorders. I again was offered an amnio which I refused and again opted for further scanning. I am now waiting to go back for me 20 week scan and I am worried about it. If the scan does pick up further slow growth then I many opt to have the amnio. Does anyone have a similiar experience with a positive outcome? As i no that trisomy 18 and trisomy 13 have very negative outcomes. Please provide any answers.