31 weeks alarming news - 10mm vantriculomegaly

Hi ladies I hope I can get some peace of mind from you sharing your knowledge and stories on this issue. 

I'm 24 years old and 31 weeks pregnant with a girl. This pregnancy was IVF due to make factor infertility. Today on a regular prenatal visit doc told me brain or ventricles I'm not even sure is measuring 10mm. He wants me to get an MRI and he drew blood work for Chromsomal issue trisomy 13 and 18 I think. He wants to r/o Down syndrome. He said for girls it should measure 7-8mm but this can be normal for her and often times is. I cannot help but worry. I have not stop crying my heart aches as this pregnancy has been the hardest thing I have ever endured. Getting pregnant was hard and it seems like there is always something wrong. Please share if you have anything. I don't know what I'll do if all the tests are positive. My heart hurts.