A dating scan is an ultrasound scan done before 14 weeks of pregnancy to help estimate your baby’s due date. In early pregnancy, most babies of the same gestational age are about the same size. A dating scan measures your baby and this helps your doctor estimate how long you have been pregnant, and when your baby is due. Everyone should have this scan done, as it’s important to have an accurate estimated due date (or EDD) for your baby so you can have the recommended tests at the right time. Knowing how far along you are is also important if your baby is born prematurely, or if you haven’t given birth by your estimated due date and you're thinking about having your labour induced.
Who needs a dating scan?
Every pregnant women need a dating scan. Most women with a regular 28-day cycle can calculate an estimated due date for their baby by counting 40 weeks from the first day of their last menstrual period. This may not be so simple or accurate in other situations, like if you have long or irregular cycles, don’t remember when you had your last period, or if you got pregnant while taking contraception that affected your cycle, or suffer from irregular menstrual cycle. Talk to our doctor and we will help you according to your need.
When is a dating scan usually done?
A dating scan is done before 14 weeks, when most babies of the same gestational age are about the same size. If your doctor or midwife thinks you should have a dating scan as well as a test for nuchal translucency, they may recommend you arrange it between 11 and 13 weeks + 6 days, so you can have both tests during a single ultrasound scan.
Who performs a dating scan?
A dating scan is usually performed by a specially-trained technician called a sonographer or by our doctor.
What happens during a dating scan?
In early pregnancy, ultrasounds including dating scans can be done through your abdomen (tummy) or vagina. The method used will depend on a few factors, including how far along your pregnancy is and your body shape.
If your scan is being done along your abdomen (known as a 'transabdominal ultrasound'), you will be asked to drink a few cups of water before you arrive so your bladder is full. This can make it easier to see inside your uterus (womb). The sonographer will apply some gel and gently move the ultrasound probe along your abdomen. It doesn’t usually hurt.
If your scan is done through your vagina (known as a 'transvaginal ultrasound'), a small ultrasound probe is lubricated and gently inserted into your vagina. The probe may be a little uncomfortable but usually isn’t painful. Scans done this way can give more detailed pictures because the probe is closer to your uterus.
Ultrasounds, including dating scans, do not harm you or your baby or increase your risk of miscarriage.
What do the results mean?
During the scan, the sonographer will measure your baby’s length from head to bottom, known as their 'crown-rump length' (CRL). This measurement can help estimate your baby’s gestational age and when it is likely to be born.
Having an accurate estimated due date is extremely important, but it’s also important to remember it’s only an estimate. Most babies are not born on their due date.
During the test, the sonographer may also:
- confirm that your pregnancy is in the right place and is not ectopic
- look for your baby’s heartbeat
- check if you are carrying more than one baby
- check that your baby’s body organs are developing normally
What is a Detailed Scan?
This detailed ultrasound scan, sometimes called the mid-pregnancy, is usually carried out when you're between 18 and 22 weeks pregnant. The scan is a medical examination. You'll be asked to give your permission for it to be carried out.
The detailed scan is offered to everybody, but you do not have to have it if you do not want to.
The scan checks the physical development of your baby, but it cannot pick up every condition.
The detailed scan is carried out in the same way as the NT scan. It produces a 2D black and white image that gives a side view of the baby. This screening test does not use 3D or colour images. 2D scan is the gold standard for detailed scan.
Make sure you understand what's going to happen, and feel free to ask any questions.
What does the scan look for?
The 20-week screening scan looks in detail at the baby's bones, heart, brain, spinal cord, face, kidneys and abdomen.
It allows the sonographer to look for 11 rare conditions. The scan only looks for these conditions, and cannot find everything that might be wrong.
- Open spina bifida
- Cleft lip
- Diaphragmatic hernia
- Serious cardiac abnormalities
- Bilateral renal agenesis
- Lethal skeletal dysplasia
- Edwards' syndrome, or T18
- Patau's syndrome, or T13
In most cases, the scan will show that the baby appears to be developing as expected, but sometimes the sonographer will find or suspect something different.
If there's a condition, will the scan find it?
Some conditions can be seen more clearly than others. For example, some babies have a condition called open spina bifida, which affects the spinal cord.
This can usually be seen clearly on a scan, and will be detected in around 9 out of 10 babies who have spina bifida.
Some other conditions, such as heart defects, are more difficult to see. The scan will find about half (5 out of 10) of babies who have heart defects.
Some of the conditions that can be seen on the scan, such as cleft lip, will mean the baby may need treatment or surgery after they're born.
In a small number of cases, some very serious conditions are found – for example, the baby's brain, kidneys, internal organs or bones may not have developed properly.
In some very serious, rare cases where no treatment is possible, the baby will die soon after they're born or may die during pregnancy.
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