Having a scan in the first few months of pregnancy can be a thrilling experience. It's amazing to see the little blob with a heartbeat that will soon become your baby. When will I have my first scan? For most mums-to-be, the first scan will be a dating scan at between 10 weeks and 13 weeks plus six days of pregnancy NICE However, the timing of your first scan will also depend on how your pregnancy is going, and where you live.
Your midwife may recommend an early scan at between six weeks and 10 weeks if you have experienced bleeding. You may also be offered an early scan if you've had a previous miscarriage. Not all units offer an early scan if all is going well in this pregnancy, though. Ask your GP if you want the reassurance of an early scan. She may be able to refer you.
Is a dating scan accurate? The scan's main purpose is to work out accurately how many weeks pregnant you are and estimate your due date.
You may have calculated the length of your pregnancy from the first day of your last menstrual period LMP. This is even though you don't actually conceive until 14 days after your LMP, or later than this if your cycle is longer than 28 days. You probably know when your LMP was, but not when you actually conceived. If you do know the conception date, this is counted as day 14, and not day one. If you have an irregular cycle, or if you've recently been taking the contraceptive pill , working out the date from your LMP isn't a very accurate method.
Scans are more accurate Bottomley et al If the scan date differs from your period dates, the scan date will be used. The scan date is usually later than the period date, so accurate dating reduces the number of women having labour induced because their babies are thought to be overdue LeFevre et al , NICE What else will the dating scan reveal?
The scan can check that your baby has a heartbeat and is developing normally. Your baby's head, limbs, hands and feet, and some organs can be seen.
Looking for abnormalities isn't the purpose of this scan, though. That's the job of your next routine scan at about 20 weeks , when more detail can be seen as your baby grows bigger. However, some major problems can be seen at your dating scan, such as problems with your abdominal wall or with your baby's skull.
You may need a second scan to check the first scan's findings. 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 gives you more time to prepare for the birth and for your doctor or midwife to plan your care. Knowing about twins early is also useful if you want to have screening for Down syndrome. This is because blood tests for Down syndrome are not very accurate if you are having more than one baby.
Instead, you will be offered a nuchal translucency NT scan. This measures a collection of fluid under the skin at the back of a baby's neck. What happens during the dating scan? The dating scan will take between five minutes and 10 minutes. Nearly all scans after 10 weeks can be done through your tummy, and most units ask you to come with a full bladder.
The sonographer will put some gel on your tummy and will move a small hand-held device a transducer over your skin to get views of your baby. If your uterus womb is very deep in your pelvis, or if you're overweight , you may be offered a vaginal scan , which can get nearer to your baby.
It shouldn't be uncomfortable, and you don't need a full bladder. You'll be given a report of the scan which will tell you exactly how many weeks pregnant you are. 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 head to bottom. This measurement, known as the crown rump length CRL , is very accurate in the first trimester. After 13 weeks your baby can curl up and stretch out, so measuring the length becomes less accurate.
Then the circumference of the head becomes the best way to measure your baby NICE At 10 weeks, your baby measures about 3cm At 12 weeks, he measures between 5cm and 6cm At 13 weeks, he measures about 7cm Can I have a screening test at the same time?
Some hospitals and private clinics offer a dating and screening scan at the same time. If you are having a screening test for Down syndrome , you'll be offered a dating scan first.
Without an accurate due date, a blood test at what you thought was 13 weeks wouldn't be helpful if you were 16 weeks pregnant when the blood was taken. It may lead to you being given a high risk factor when your baby is actually developing normally false positive result NICE , Gray and Ward Nuchal translucency can be measured from 11 weeks to 13 weeks plus six days, or when the head-to-bottom measurement CRL is 45mm to 84mm.
This estimates the risk of Down syndrome and other chromosomal abnormalities. A combined NT scan and blood test gives a more accurate diagnosis than an NT scan alone.
This is available in most parts of Australia. Are there any disadvantages to having a scan? Sometimes, having a scan can cause unnecessary worry. It may show a very minor problem, or something that may get better on its own. As with all screening tests, there can be false positive and false negative results. For example, about one in 20 women will appear to be at high risk, from the nuchal translucency scan to assess the risk of Down syndrome.
However, most of these babies will turn out not to have Down syndrome. It is entirely up to you whether or not you have a scan. You could have a dating scan, without NT screening, or even change your mind on the day of your scan.
Your sonographer will support your decision, either way. It's worth knowing that there's no evidence that scans are unsafe for you or your baby.
Don't forget to download our free app for a day-by-day guide to your pregnancy. Dating and growth in the first trimester.
A randomized trial of prenatal ultrasonographic screening: Am J Obstet Gynecol. Fetal Anomaly Screening Programme. Consent standards and guidance for Down's syndrome and Fetal Anomaly screening. National Institute for Health and Clinical Excellence, clinical guideline Routine care for the healthy pregnant woman.
Royal College of Obstetricians and Gynaecologists. Antenatal Screening for Down's syndrome, setting standards to improve women's health. UK National Screening Committee.
The rhetoric of "natural" in natural childbirth: Soc Sci Med 59 7: