In the late 1950s, using the knowledge of sonar and radar that he acquired in the Royal Air Force, Professor Ian Donald in Glasgow, Scotland, pioneered the use of ultrasound in pregnant mothers. In those days it was only possible to take very basic measurements of the fetal head. Now ultrasound can provide the detailed examination of almost all fetal structures, and has become a useful diagnostic tool.
What an ultrasound scan involves
To start with, the person doing the scan will rub a special gel onto your abdomen to reduce signal loss. He or she will then place a probe (a transducer) onto your abdomen. The transducer both emits and receives ultrasound waves. These reflected ultrasound waves are continuously assembled into a picture on a screen. This shows solid structures as white and liquid structures as black. Moving structures, such as the fetal heart, can be assessed, and freezing the picture enables accurate measurements of the fetus to be taken. In the first 3 months of pregnancy, you will need to have a full bladder before a scan in order to improve image quality. An internal scan using a transvaginal probe can be helpful in early pregnancy.
Uses of ultrasound scans
Ultrasound has been shown to be safe, it has become widely acceptable to mothers and an indispensable part of antenatal care. Ultrasound scans are used for a number of purposes, as well as those described below. monitoring fetal growth and wellbeing
Until 14 weeks, the size of your baby is measured by his crown-rump length. After this, fetal weight can be estimated by measuring the head circumference (HC), biparietal diameter (BPD - the width of the head), abdominal circumference (AC), and femur length (FL - length of the thigh bone). If necessary, growth measurements will be taken every 2 weeks. The amount of amniotic fluid is also important. A low amount may indicate that your baby is not getting enough nutrients.
Locating the placenta
Ultrasound is used to locate the placenta. If the placenta is too low down and remains that way in late pregnancy, you may need a caesarean section.
Guiding invasive procedures
Procedures such as chorionic villus sampling and amniocentesis are best performed under continuous ultrasound control. In this way the needle can be safely guided to the sampling site without harming you or your baby.
Ultrasound scans in early pregnancy (5-14 weeks)
It is possible to diagnose pregnancy from about 4-5 weeks after your last menstrual period using ultrasound. Detection of a fetal heart beat at 5-6 weeks will confirm the viability of your pregnancy. Ultrasound is therefore extremely useful if you experience vaginal bleeding during early pregnancy because it often makes it possible to determine the site and viability of your pregnancy.
If you are unsure of your dates, an ultrasound scan in early pregnancy is particularly useful. Later in pregnancy, dating becomes progressively less accurate. An early scan can confirm a multiple pregnancy and determine the number of placentas. Also, between 11 and 14 weeks, it can screen the fetus for chromosomal abnormalities, such as trisomy 21, by measuring the thickness of the fat pad at the back of the neck.
Detailed scan at 18-20 weeks
You may be offered a scan to look for any abnormalities in your baby. This scan carefully examines each part of your baby in turn and any problems are noted. The scan will detect even small abnormalities, such as a cleft lip or tiny holes in your baby's heart. Sometimes a scan shows an abnormality which, by itself, is of little significance but which may be associated with an underlying problem, such as an abnormal number of chromosomes (for example, Down's syndrome).
Such findings are called'soft markers'and include: too much fluid on the kidney (hydronephrosis), short femurs, bright areas within the heart (echogenic foci) and a bright appearance of the fetal bowel (echogenic bowel). Although the presence of a soft marker may indicate an increased risk of chromosome abnormality, this is not as common as previously thought.
Colour doppler ultrasound
The use of a colour Doppler converts the movement of structures (for example, blood moving around the fetus) into different colours on the screen. This allows particular blood vessels to be examined to see how much blood is flowing through them.
For example, both the umbilical cord and the uterine arteries can be visualized and blood flow can be measured. A high resistance flow in the umbilical arteries may indicate a problem with the baby, while a similar measurement in the uterine arteries suggests an increased risk of the mother developing pre-eclampsia.