First Trimester Screening: Complete Guide
First trimester screening is offered to every pregnant woman on the NHS and gives you and your midwifery team an early picture of how your baby is developing, along with an assessment of the chance of certain chromosomal conditions. It's entirely optional, but most people choose to take it up.
What first trimester screening involves
The screening combines an ultrasound scan with a blood test, usually carried out between 11 and 14 weeks of pregnancy.
- Dating scan — confirms how many weeks pregnant you are based on your baby's measurements, checks that the pregnancy is developing in the expected place, and confirms whether you're expecting one baby or more.
- Nuchal translucency (NT) measurement — measures the small pocket of fluid at the back of your baby's neck, which is taken as part of the same scan appointment.
- Combined blood test — checks levels of two pregnancy-related proteins, hCG and PAPP-A, usually taken around the same time as the scan.
- Combined result — your NT measurement, blood results, and your age are combined into a single "chance" result (for example, a 1 in 2,000 chance) for conditions including Down's syndrome, Edwards' syndrome, and Patau's syndrome.
What the results mean
Combined screening gives you a chance, or probability, rather than a yes-or-no answer — it is not a diagnostic test. A "lower-chance" result means the likelihood is small but not zero, while a "higher-chance" result means the calculated probability has passed a certain threshold and further discussion is recommended. If you receive a higher-chance result, your midwife or a specialist will talk you through what it means and what further tests, such as NIPT, CVS, or amniocentesis, could give you more definite information if you want it.
Deciding whether to have screening
This is a personal decision, and there's no obligation to take up any part of it.
- You can accept the dating scan but decline the NT measurement and blood test if you'd simply like confirmation of dates and development.
- You can decline all screening and still receive full antenatal care — this won't affect the care you're offered.
- It can help to think in advance about what you would do with a higher-chance result, as this can make the decision about whether to test easier.
- Your midwife is there to answer questions without pressuring you either way, so it's worth raising anything you're unsure about at your booking appointment.
Frequently Asked Questions
Is first trimester screening compulsory?
No, screening is entirely optional and you're free to accept, decline, or choose only part of it, such as having the dating scan without the blood test. Your care team will support whatever decision you make and it won't change the standard of antenatal care you receive.
What happens if I get a higher-chance result?
A higher-chance result doesn't mean your baby definitely has a chromosomal condition — it means the calculated probability is higher than the standard threshold, and you'll be offered a conversation with a specialist about further, more definitive testing if you want it. Many people who receive a higher-chance result go on to have a straightforward, unaffected pregnancy.
Can first trimester screening tell me my baby's sex?
No, the dating and NT scan is focused on measuring development and calculating chance results, not on determining sex, and sonographers in early pregnancy scans don't routinely report this. Sex is usually confirmed at the mid-pregnancy anomaly scan around 20 weeks, if you choose to find out.
This article is for informational purposes only and does not constitute medical advice.
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