Don't skip the test... First trimester screening test should be done between 11-13wks  which comprises sonography for NT and two bld test.. It's also called double marker test.. It's noninvasive test and will assess the risk of syndromic mental retardation chance in fetus.. As such changes or risk is age related also so at your age  it's mandatory... If you refuse to doing it doctor will safly write on your paper  .if this test normal  then no need to do CVS  testing. Now it's up to you... I advise better to do