The 12-week scan is one of the most important ultrasound examinations of pregnancy. For many parents, it is the first clear view of their baby and the moment the pregnancy is dated accurately.
Traditionally, the scan has been associated with dating and screening for Down's syndrome. With modern ultrasound and fetal medicine expertise, a detailed examination at 12–13 weeks can tell you considerably more: about your baby's early anatomy and heart, the placenta, twin pregnancies and the risk of complications such as pre-eclampsia.
Key points
- A detailed first-trimester assessment is performed between 11 weeks and 13 weeks 6 days, when the baby measures about 45–84 mm.
- Beyond dating and screening, it can identify several major structural and heart abnormalities weeks before the 20-week scan.
- It can also estimate the risk of preterm pre-eclampsia and establish chorionicity in twins, so the right care can be planned early.
- A normal scan is reassuring, but the 20-week anatomy scan remains essential.
When is the 12-week scan performed?
The routine NHS dating scan is offered between 11 and 14 weeks of pregnancy. A detailed first-trimester assessment, including nuchal translucency and combined screening, is performed between 11 weeks and 13 weeks 6 days, when the baby's crown–rump length measures approximately 45–84 mm.
At this stage the baby is large enough for many structures to be examined, while the markers used for first-trimester screening can still be measured reliably.
The routine scan vs a detailed first-trimester assessment
The routine NHS scan is designed to confirm that the pregnancy is progressing, check the heartbeat, identify twins, establish the due date and, if you choose, offer combined screening for Down's, Edwards' and Patau's syndromes. A systematic examination of the baby's anatomy is not its principal aim, and the detail varies with local pathways, image quality and the experience of the examiner.
A detailed first-trimester assessment builds on these aims. Its purpose is not only to confirm that the pregnancy is progressing, but to obtain as much reliable information as possible, at an early stage, about your baby's development and your individual pregnancy risks. It complements, rather than replaces, routine NHS antenatal care and the mid-pregnancy anatomy scan.
What our first-trimester assessment includes
At The Pregnancy Clinic, every first-trimester scan is performed by a consultant in fetal medicine, following Fetal Medicine Foundation standards. Alongside dating and the baby's wellbeing, the assessment includes:
Nuchal translucency and additional genetic markers
Nuchal translucency (NT) is measured with care, together with the nasal bone and blood flow in the ductus venosus and across the tricuspid valve. Assessed systematically, these markers improve screening for chromosomal conditions and reduce false alarms. Read about nuchal translucency.
Early fetal anatomy
A systematic examination of the developing brain, face, spine, abdomen, bladder and limbs, which can identify several major structural abnormalities weeks before the 20-week scan. Read about early anatomy.
The fetal heart assessment
Using high-resolution imaging and colour Doppler, many major congenital heart defects can be suspected or diagnosed at this stage in experienced hands. Read about the heart at 12 weeks.
The placenta
Its position and appearance are assessed and, after a previous Caesarean section, its relationship to the scar, so that conditions such as placenta accreta spectrum can be suspected early in selected pregnancies.
Pre-eclampsia screening
Combining your history, blood pressure, uterine artery Doppler and blood markers gives an individual risk of preterm pre-eclampsia. Women at high risk benefit from aspirin started at 11–14 weeks. Read about pre-eclampsia screening. Read about uterine artery Doppler.
Twin pregnancies
The scan establishes whether twins share a placenta (chorionicity) and an amniotic sac (amnionicity). This determines the type of monitoring needed for the rest of the pregnancy, as twins sharing a placenta face additional risks such as twin-to-twin transfusion syndrome.
Screening for Down's syndrome: combined test or NIPT?
Combined screening brings together your age, the nuchal translucency and two blood markers, PAPP-A and free β-hCG. In the NHS, it identifies around 85–87% of pregnancies affected by Down's syndrome. Including the additional ultrasound markers and a detailed anatomy assessment can improve its performance.
NIPT analyses placental DNA in the mother's blood and detects around 99–99.7% of pregnancies affected by Down's syndrome, with a false-positive rate of about 0.04–0.1%. It remains a screening test, and it does not examine the baby's anatomy, so a detailed scan is still recommended alongside it.
For further information, read: NIPT vs Combined Screening: Which Test Should I Choose?
Why early information matters
When development appears normal, a detailed 12-week scan offers valuable early reassurance. Where a concern is identified, parents have more time for genetic investigations, specialist counselling and individualised planning, rather than waiting until the middle of pregnancy for the first detailed look at their baby.
Does a normal 12-week scan mean everything is normal?
A normal detailed scan is reassuring, but it cannot exclude every condition. Some abnormalities only become apparent as the baby grows, and some genetic conditions have no visible signs. The fetal anatomy scan at 20 weeks therefore remains an essential part of pregnancy care.
Related pregnancy guides
Frequently asked questions
When is the 12-week scan done?
The NHS dating scan is offered between 11 and 14 weeks. A detailed first-trimester assessment with nuchal translucency is performed between 11 weeks and 13 weeks 6 days.
What is the difference between the NHS 12-week scan and a detailed first-trimester scan?
The NHS scan focuses on viability, dating, the number of babies and offering screening. A detailed assessment adds a systematic examination of the baby's anatomy and heart, additional genetic markers, and can include pre-eclampsia screening and specialist twin assessment.
Can abnormalities be detected at the 12-week scan?
Yes. In experienced hands, several major structural and heart abnormalities can be identified at 12–13 weeks. Some conditions only become apparent later.
Can the 12-week scan predict pre-eclampsia?
Yes. Combining your history, blood pressure, uterine artery Doppler and blood markers estimates the risk of preterm pre-eclampsia, and women at high risk can benefit from aspirin started at 11–14 weeks.
Why is the 12-week scan important in twins?
It establishes whether the twins share a placenta and amniotic sac, which determines how closely the pregnancy needs to be monitored.
Does a normal 12-week scan mean everything is normal?
A normal detailed scan is reassuring but cannot exclude every condition. The fetal anatomy scan at 20 weeks remains essential.
First-trimester pregnancy care at The Pregnancy Clinic
At The Pregnancy Clinic, we provide consultant-led first-trimester ultrasound examinations with a focus on detailed assessment of fetal development and individual pregnancy risks.
Our approach extends beyond pregnancy dating and routine chromosomal screening to include comprehensive assessment of early fetal anatomy, examination of the developing heart and, where clinically appropriate, evaluation of placental and maternal factors that may influence pregnancy care. We aim to provide clear explanations, personalised counselling and sufficient time to discuss the findings and any further investigations that may be recommended.
Book a first-trimester scan →An early fetal anatomy scan complements routine NHS maternity care and does not replace the recommended mid-pregnancy fetal anatomy scan. The Pregnancy Clinic provides planned appointments only; for urgent concerns, please contact your local NHS maternity unit, Early Pregnancy Unit, NHS 111 or your GP.
Clinical references
- Akolekar R, Syngelaki A, Poon L, Wright D, Nicolaides KH. Competing risks model in early screening for preeclampsia by biophysical and biochemical markers. Fetal Diagn Ther. 2013;33(1):8–15.
- Minnella GP, Crupano FM, Syngelaki A, Zidere V, Akolekar R, Nicolaides KH. Diagnosis of major heart defects by routine first-trimester ultrasound examination: association with high nuchal translucency, tricuspid regurgitation and abnormal flow in the ductus venosus. Ultrasound Obstet Gynecol. 2020;55(5):637–644.
- Syngelaki A, Hammami A, Bower S, Zidere V, Akolekar R, Nicolaides KH. Diagnosis of fetal non-chromosomal abnormalities on routine ultrasound examination at 11–13 weeks’ gestation. Ultrasound Obstet Gynecol. 2019;54(4):468–476.
- Rolnik DL, Wright D, Poon LC, O’Gorman N, Syngelaki A, de Paco Matallana C, Akolekar R, et al. Aspirin versus placebo in pregnancies at high risk for preterm preeclampsia. N Engl J Med. 2017;377(7):613–622.
- Gil MM, Accurti V, Santacruz B, Plana MN, Nicolaides KH. Analysis of cell-free DNA in maternal blood in screening for aneuploidies: updated meta-analysis. Ultrasound Obstet Gynecol. 2017;50(3):302–314.
- Bilardo CM, Chaoui R, Hyett JA, Kagan KO, Karim JN, Papageorghiou AT, et al. ISUOG Practice Guidelines (updated): performance of 11–14-week ultrasound scan. Ultrasound Obstet Gynecol. 2023;61(1):127–143.
- NHS England. Fetal anomaly screening programme handbook: screening for Down’s syndrome, Edwards’ syndrome and Patau’s syndrome.
- National Institute for Health and Care Excellence. Twin and triplet pregnancy (NG137). 2019.
- NHS. 12-week scan.