Anomaly scan
A detailed head-to-toe examination of the baby's anatomy — brain, heart, spine, face, abdomen, kidneys and limbs — plus the placenta and fluid.
When: Usually 18 to 22 weeks
Examination — looks at how things are
Also called: Detailed scan · 20-week scan · Mid-trimester scan · TIFFA scan
What it is
This is the longest and most detailed scan of the pregnancy. The sonographer works systematically through the baby's anatomy: the brain and skull, the face and lips, the spine, the chest and heart, the abdominal wall and organs, the kidneys and bladder, and the arms and legs.1
It also checks things around the baby — where the placenta is lying, how much fluid there is, and how the baby is growing against the dates set earlier.
It is a careful look, not a guarantee. Some conditions are not visible on ultrasound at all; others develop later in pregnancy or are simply too subtle to see. A reassuring anomaly scan is genuinely reassuring, and it is not a promise that nothing will ever be found.1
Why 18 to 22 weeks
It is the window where the baby is developed enough to examine and still small enough to see well. Earlier, the organs are too small to assess properly. Later, the baby is bigger and more cramped, bones cast more shadow, and views become harder to obtain — so a scan done too late may be a worse scan, not just a later one.1
Things outside your control affect the view
How the baby is lying, how much fluid there is, and body habitus all change how much can be seen. If the views are not adequate you may be asked to come back, or to walk around and return in twenty minutes. This is routine and is not, by itself, a sign that something is wrong.
Checking the placenta matters as much as the baby
The scan records where the placenta is sitting. A placenta lying low near the cervix is common at this stage and usually moves up as the womb grows, but it is worth knowing about, and a follow-up scan later in pregnancy checks whether it has.
Finding out the sex
The sex can often be seen at this scan, though it is not what the scan is for and is never guaranteed — it depends entirely on how the baby is lying. Tell the sonographer at the start if you would rather not be told.
No scan or screening test can rule out every condition. What each one looks at, what it found, and what it means for you is a conversation with the doctor who performed it — this page explains the tests, it does not interpret your results.
References
- Salomon LJ, Alfirevic Z, Berghella V, Bilardo CM, et al.. ISUOG Practice Guidelines (updated): performance of the routine mid-trimester fetal ultrasound scan. Ultrasound in Obstetrics & Gynecology. 2022;59(6):840-856. PMID 35592929. doi:10.1002/uog.24888The international guideline on the mid-trimester anomaly scan — when it is performed, what is examined, and what it can and cannot detect.
Sources are listed for the specific statements they support. They are published research and guidance, not a substitute for a consultation — what applies to you is decided by a doctor who has assessed you.