This is related to your baby’s chromosomes, and while it could be nothing, it can indicate that your baby is at higher risk of being born with Down’s Syndrome. Generally, your doctor will refer you to a specialist, or will perform further testing for other markers of Down’s Syndrome. They will also review your test result for Down’s Syndrome, and your previous ultrasound results.
Generally speaking, if all your tests are normal, and you’re under 35 years old, you shouldn’t have too much to worry about. Likewise if you’ve had an amniocentesis and that returned a normal result. If you are concerned, or you fall into a high risk category for Down’s Syndrome, then it’s best to speak to your doctor for clarification.