What is it?

Ultrasound scans are a way to check your pregnancy is progressing healthily by looking at your baby while it is still growing inside you.

How?20wk scan pic

You lie on a couch and the sonographer will squirt cold jelly onto your bump, then press on your tummy with a small wand that looks a bit like a microphone. The wand emits high-frequency sound waves which bounce (echo) off different tissues and fluids to build a picture of the baby in the womb (uterus).

Bone, being hard, reflects the sound waves the most, and so the echo from bone is shown as white on the scan. In later scans the baby’s skull, ribcage, spine, arms and legs, even fingers and toes can be seen as white marks and lines. The amniotic fluid which surrounds your baby returns no echo at all, and so it appears black on the scan. The rest of the tissues return some echo and so these show up as grey.

The machine displays this information as a picture the sonographer can interpret to check that everything is developing as it should. Often you will have a second smaller screen facing you so you can see what the sonographer sees. The sonographer can also use the equipment to take measurements and even to listen to the flow of blood in the umbilical cord or the baby’s heart.


Women are usually offered their first scan between 8-14 weeks, at which point the fetus is only a few millimetres long.

This is often called the ‘dating’ scan as the size of the fetus at this stage is used to determine its age, and your likely due date (EDD) will be revised to fit with the scan results.

This would also be when you would find out if you’re pregnant with more than one baby!

10wk scan picAt this stage, they usually look like a little bean and it can be difficult to tell one end from the other.

You will then be offered a second scan somewhere around 18-21 weeks. This is usually known as the ‘anomaly’ scan, as the purpose of it is to check for any issues (anomalies) which might indicate a problem.

The sonographer will often measure the baby’s head, abdomen and thigh bone as a way of estimating the baby’s birth weight, however not all hospitals do this as it has been shown not to be reliably accurate.

If you are classed as obese then you will likely be offered extra scans as you near the end of your pregnancy. This is because bigger women are associated with having both larger babies (macrosomia) and babies that seem to stop growing in the last few weeks and may need to be delivered early.

Whether you qualify for extra scans depends on the particular policy of your hospital, as different hospitals choose different BMI (body mass index) points as a trigger for scans, and schedule them at different times. See the policies page for details of your hospital trust policy on BMI to find out more.


The anomaly (18-21 week) scan may also disclose the gender of your baby. This is not guaranteed, however, as it does depend on the position the baby is lying in, and sometimes they are a little shy.

Also, some hospitals have a policy of non-disclosure of gender due to concerns that parents may terminate babies they consider to be the ‘wrong’ gender.

Why Not?

Other than a bit of pressure on your bump, scans are painless. There have been no reported side effects on mothers or babies in the decades of using them and they can be carried out at any stage of pregnancy. They can offer an invaluable insight into your baby’s development and spot problems at an early stage.

But, like all treatments, scans are offered to you and it is entirely at your discretion to accept or refuse them.

You may find, as a bigger mum, that ‘limited visibility due to maternal habitus’ or ‘image suboptimal due to maternal physique’ or similar is recorded on your scan notes. This is hospital-speak for couldn’t see very well because the mum is too fat. When questioned about this however, it seems that women are reporting this being added to their notes as a matter of routine; some sonographers have conceded that they could see perfectly well everything they needed to, but hospital policy is to write the disclaimer, apparently to cover the hospital in the event of an issue coming to light that should have been picked up by the scan.

Hmmm. Feel free to question and challenge such additions to your notes if the sonographer didn’t appear to be having any problems. Whether or not there is a problem with visibility often depends on where your adipose (fat) tissue is located. If there is a lot of fat tissue on your bump, the ultrasound waves have more to travel through and so the signal the wand receives back is weaker.

If there is a problem with visibility, there may be the option of performing a vaginal scan, in which a special wand is inserted into the vagina (once it has been covered with a condom and lubricant) and the ultrasound waves are passed that way instead, although this is usually only useful in the first trimester.

While scans can be very useful at spotting problems, they can be the cause of unnecessary intervention when the measurements are inaccurate. As reported by the NHS in the link about miscarriage above, even in a very tiny fetus the variation in measurements between different sonographers was calculated as plus or minus 18.78%, which meant that 20mm by one examiner could be read as a measurement of between 16.8mm and 24.5mm by other examiners.

Estimated birth weight is arrived at as a function of four different measurements – two head measurements, abdomen circumference, and femur (thigh bone) length. As with any mathematical function where multiplication is involved, tiny errors can result in very large discrepancies. It is not uncommon for women to be told they are having a ‘big’ or ‘small’ baby, and for its weight to be decidedly normal when delivered, or even the opposite.

Any intervention proposed purely on the grounds of estimated birth weight should be carefully considered.


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1 thought on “Scans”

  1. Thank you so much for this website, can’t believe I’ve only just come across it at 37 weeks. The sonographer at our anomaly scan had.put ‘difficult scan’ next to my bmi. Only problem she had was baby kept wriggling away from where she had the scanner, photos were very clear. Wish I’d have known to complain about it, none of my other 5 scans have had any such comment.

    I’ll also be fighting for my waterbirth at hospital if I decide against a homebirth when time comes. I got told on friday I wouldn’t be allowed to have one, despite my consultant not having an issue with it.

    My body, my birth, my choice.

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