What Actually Happens at Your 20-Week Scan

The NHS offers every pregnant woman in England a mid-pregnancy scan between 18 and 21 weeks, usually called the "20-week scan" or the anomaly scan. It's the longest scan you'll have — often 20–45 minutes — because the sonographer is working through a detailed checklist of your baby's organs and structures, not just confirming a heartbeat.

What the sonographer is actually checking

The scan follows a standardised list covering the baby's head and brain, face, spine, heart, stomach, kidneys and bladder, arms, legs, hands and feet, and the position and health of the placenta. It also confirms your due date more precisely and checks the amount of amniotic fluid. It is not primarily a "meet your baby" scan, even though most parents leave with a photo — it's a structured medical screen for 11 specific rare conditions, from spina bifida to cleft lip to certain heart conditions.

The scan can't tell you everything

It's worth knowing the limits going in: the scan detects most, but not all, physical conditions, and it says nothing about a baby's future development, intelligence, or most genetic conditions (those are covered by earlier combined/quad screening, not this scan). Some structures — particularly parts of the heart and spine — can be hard to see clearly if your baby is in an awkward position, which is why you may be asked to walk around and come back, or booked for a rescan.

"Soft markers" — the phrase that causes the most worry

Sometimes a sonographer notes a minor variation — a bright spot on the heart, slightly wide kidneys, a shortened thigh bone — called a "soft marker." These are common, often present in perfectly healthy babies, and are not diagnoses. Depending on which marker and your individual risk factors, you may simply be told and reassured, or offered an additional scan or discussion with a fetal medicine specialist. A soft marker on its own is rarely a cause for serious concern, but it's a completely reasonable thing to ask the sonographer to explain in plain terms before you leave.

Finding out the sex

Most NHS trusts will tell you the likely sex if you ask and it's clearly visible, though it isn't part of the official checklist and isn't guaranteed to be accurate or offered everywhere — policies vary by trust, so it's worth asking your midwife in advance what your hospital's approach is.

If something is flagged

If the sonographer finds something that needs a closer look, you'll usually be seen quickly by a fetal medicine consultant, often the same day or within a few days — not left waiting anxiously for weeks. Try to remember that "needs a closer look" and "there's a problem" are not the same thing; a large proportion of referrals from this scan lead to reassurance once a specialist has looked in more detail.

Practical tips for the appointment

• You'll usually be asked to have a comfortably full bladder — check your hospital's specific instructions, as this varies.
• Most units allow one partner or support person in with you; check in advance if you want to bring anyone else.
• Wear something that makes it easy to access your lower abdomen — a two-piece rather than a dress.
• Write down any questions beforehand; it's easy to forget them once you're looking at the screen.