During your first trimester there are two types of scan that are performed. These are:
Dating Scan
Nuchal Translucency Scan
Having a scan in the first few weeks of pregnancy can be thrilling experience. It is a wonderful thing to see a little blob with a heartbeat that will become your baby.
Dating Scan (10-14 weeks)
All pregnant women will be offered a dating scan at 10-14 weeks of pregnancy.
This scan is recommended before you have a screening test for Down’s syndrome, because having an accurate due date makes the test more reliable. This is because the hormones measured in the blood test vary at different stages of the pregnancy, so what might be normal at 14 weeks would not be normal at 16 weeks.
If the blood test is taken too early because your dates are wrong then you may get a false positive, which is when you are given a higher risk factor when really there is nothing wrong.
Can the dating scan tell exactly how many weeks pregnant I am?
The purpose of the dating scan is to work out how far along you are, and to estimate your due date. The length of a pregnancy is calculated from the first day of your last menstrual period (LMP), even though you don’t conceive until 14 day after your LMP, and later than this if your normal menstrual cycle is greater than 28 days. The LMP is used because most women know when their last period was, but not many know when they actually conceived. If you do kow the conception date, this is classed as day 14 and not day one.
If you have an irregular menstrual cycle, or you’ve been taking the contraceptive pill, working out the date of your LMP isn’t a very accurate method. Scans are more accurate than using the LMP and, if the scan date differs from your period dates then the scan date will be used.
The scan date is usually later than the period date so accurate dating has been found to reduce the number of women having labour induced because their baby is overdue. It also helps you avoid the disappointment you may feel when your baby is ‘late’.
What else will the dating scan reveal?
During this scan the sonographer can check that your baby has a heartbeat and is developing normally. The baby’s head, limbs, hands and feet and some organs can be seen at this early stage. You should still have a scan at about 20 weeks because more detail can be seen as the baby grows bigger.
The dating scan will also show if you’re expecting twins, triplets or more. It’s helpful to know about twins early on, as it’s easier to see whether or not they share a placenta. Finding out about having twins early in pregnancy also you gives you more time to prepare for the birth and to plan your care.
Knowing about twins early is also useful if you want to have a screening test for Down’s syndrome. This is because the usual test for Down’s syndrome is a blood test at about 16 weeks, and this test doesn’t work very will when there is more than one baby. Instead you will be offered a nuchal translucency scan.
What happens during the scan?
The scan will take around 5 to 10 minutes. Nearly all scans after 10 weeks can be done through your tummy, and most units will ask you to come with a full bladder. The sonographer will put some gel on your tummy and will move a small hand-held transducer over your skin to get views of the baby.
If your womb is very deep in your pelvis or if you’re overweight, you may be offered a vaginal scan which can get nearer to the baby. It shouldn’t be uncomfortable, and a big advantage is that you don’t need a full bladder.
You will be given a report of the scan which will tell you exactly how many weeks you are. It’s a good idea to keep it with your maternity notes so your midwife and anyone else involved in your care can see it.
How big will my baby be at the dating scan?
Your baby is measured from its head to its bottom. This measurement, known as the Crown Rump Length, is very accurate in the first trimester. After 13 weeks the baby can curl up and stretch out, so measuring the length becomes less accurate. The width of the head then becomes the best way to measure the baby.
- At 10 weeks in measures 3 cm
- At 12 weeks it measure 5-6 cm
- At 13 weeks it measure 7 cm
Can I have a nuchal scan at the same time are the dating scan?
Nuchal translucency can be measured from 11 – 14 weeks to estimate the risk of Down’s syndrome and other chromosomal abnormalities.
Some hospitals offer a nuchal translucency scan to all women at 11-14 weeks, but as this isn’t available everywhere at present, you might want to consider having it done privately.