ultrasound scans
you will be offered one or two routine ultrasound scans in the first half of pregnancy (i.e. usually by 20 weeks ). as with blood tests, it is up to you to decide whether you want any scans to be performed in your pregnancy. the scientific evidence is that ultrasound scanning during pregnancy is safe for mother and baby.
it is important to be aware of what the scans are intended for.
most scans fall into one of three categories:
- early scans to check the number of babies and to date the pregnancy
- anomaly scans, recommended to be done at about 20 weeks
- scans later in pregnancy, not done routinely but when there are doubts
about the baby's growth and wellbeing, or about the position of the placenta
reasons for scans
dating pregnancies. it is important to know the age of the baby in the womb so that we know how mature the fetus is. scan dates are more accurate than menstrual dates if done before 22 wks. this is because they look at the actual age of the fetus, whereas menstrual dates are based on the first day of the last menstrual period and assume that fertilization has occurred 14 days later, which is not always the case. please note also that most babies are not born on their actual due date, but during a 4 week period around it. usually babies come when they are ready.
early pregnancy ultrasound scan. it is standard to offer you an early scan between 10 weeks and 13 weeks and 6 days to confirm the pregnancy and the number of babies in the womb, and to calculate the date of delivery. some units check for nuchal translucency at this time, as one method to screen for down's syndrome.
anomaly scan. a detailed scan is usually offered at 18-21 weeks gestation. this is a good time to check for abnormalities (anomaly ) of the head, spine, limbs, abdomen and heart of the baby. however, it is important to understand that ultrasound will not identify all problems. detection rates will vary depending on the type of anomaly, the position the baby is lying in, previous surgery to your abdomen and maternal size. if a problem is suspected you will be referred to a specialist to discuss the options available to you. sometimes problems are suspected in normally developing babies. list of the most common anomalies and the chance of it being identified on ultrasound is
% %
anencephaly 97 - 100
spina bifida 61 - 92
major cardiac anomalies
(hypoplastic ventricle )14 - 61
diaphragmatic hernia 30 - 62
gastroschisis 86 - 100
exomphalos 45 - 92
major renal tract problems
(renal agenesis ) 76 - 85
scans to check the baby's well-being later in pregnancy. an ultrasound scan can check what size the baby has reached at a particular point in pregnancy. the main measurement for this is the abdominal circumference, which includes the size of the liver (the main nutritional store of the growing baby ) and the abdominal wall thickness (related to fat reserves ). scans to measure fetal size are usually not done routinely, but if there is a suspicion that the baby may not be moving or growing well. an assessment of the amount of amniotic fluid (liquor ) around the baby is also important, as low liquor is linked to fetal growth restriction and can cause fetal distress. if the scan suggests that the baby may be small, doppler flow studies can establish whether the baby is receiving sufficient blood flow with oxygen and nutrients through the placenta (afterbirth ). scans are sometimes also done to examine and identify the position of the placenta, which may have been low in the womb at an earlier scan. a low placenta increases the risk of heavy bleeding later in pregnancy.
sex of the baby. although the scan can sometimes tell whether the baby is a boy or girl, this is not always accurate. please note, scans are not done to find out what the sex of the baby is, unless requested by the parents.
