Combined Screening Test: Complete Guide
The combined screening test is the standard NHS screening offered to every pregnant woman in the UK during the first trimester, and understanding what it actually measures — and what the results mean — can take a lot of the anxiety out of waiting for your appointment.
What the test actually involves
"Combined" refers to the fact that three separate pieces of information are brought together into a single risk calculation, rather than any one test being decisive on its own.
- The nuchal translucency (NT) measurement — an ultrasound scan, usually done as part of your 12-week (dating) scan between roughly 11 and 14 weeks, measuring the small pocket of fluid at the back of your baby's neck. A slightly larger measurement is associated with a higher chance of certain chromosomal conditions, though it's a probability marker, not a diagnosis.
- A blood test — measuring levels of two pregnancy-related hormones, hCG and PAPP-A, both of which tend to sit at different average levels when certain chromosomal conditions are present.
- Your personal details — your age, weight, and how far along you are all factor into the final calculation, since baseline risk naturally varies with maternal age.
These three elements are combined into a single number expressing your baby's chance of having Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), or Patau syndrome (trisomy 13).
Understanding your result
Results are expressed as odds, such as "1 in 500" or "1 in 100," representing your baby's calculated chance of having one of these conditions — not a yes/no answer.
- A "lower chance" result (sometimes still called "low risk") typically means your calculated odds are below the national screening threshold of 1 in 150. This is reassuring, though it doesn't guarantee a condition is entirely absent — screening reduces likelihood, it doesn't eliminate it.
- A "higher chance" result means your odds come out at 1 in 150 or higher. This doesn't mean your baby definitely has the condition — most babies in this category do not — but it does mean you'll be offered further options.
- If you receive a higher-chance result, you'll typically be offered NIPT (non-invasive prenatal testing, a more accurate blood test) or a diagnostic test such as CVS or amniocentesis, which can confirm a diagnosis but carry a small risk of miscarriage. Your midwife or a specialist counsellor will talk you through the options without any pressure to proceed.
Deciding whether to have the test
Screening is entirely optional in the UK, and it's a genuinely personal decision.
- Some parents want as much information as possible to prepare, plan care, or make informed decisions about the pregnancy.
- Others prefer not to know and decline screening altogether, which is a completely valid choice and won't affect the standard of care you receive.
- If you're unsure, it can help to think in advance about what you would actually do with a higher-chance result, since that's often the more useful question than the test itself.
Frequently Asked Questions
What does "low risk" (lower chance) mean?
It means your calculated odds came out below the national threshold of 1 in 150, which is reassuring for most parents. It doesn't mean zero risk — no screening test can offer that — but the vast majority of women receive this result, and no further testing is required unless you want it.
What does "high risk" (higher chance) mean?
It means your calculated odds are 1 in 150 or higher, so you'll be offered further options such as NIPT or a diagnostic test like CVS or amniocentesis. Importantly, most babies in the higher-chance category do not have the condition being screened for — it's a signal to gather more information, not a diagnosis.
Can I decline the combined screening test?
Yes, absolutely. Screening is optional and entirely your choice — declining it will not affect the quality of your antenatal care in any way. Your midwife can talk through the pros and cons if you're undecided, without any pressure either way.
This article is for informational purposes only and does not constitute medical advice.
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