Conslutant in Foetal Medicine
MBBS MRCOG (UK)
Diploma in Obstetric Ultrasound (RCOG, UK)
Fellowship in Foetal Medicine (FMF, UK)
What is sonography? Why is it required during pregnancy? Dr. Prathima, a foetal medicine specialist and a sonologist, answers commonly asked questions on this procedure.
What is sonography?
Sonography, Ultrasound, Scan - all terms imply the same thing.
Prior to World War II, sonar, the technique of sending sound waves through water and observing the returning echoes to characterize submerged objects, inspired early ultrasound investigators to explore ways to apply the concept to medical diagnosis. This principle of passage and reflection of 'sound' waves of higher wavelengths than what the human ear can hear, through media of different densities, is now used medically to visualise internal organs of the body.
What is internal sonography, and when is it required?
'Internal sonography' is the same as a 'transvaginal' or 'vaginal' scan. A thin probe is introduced into the vagina to enable better visualisation of deeper organs. It is usually required to visualise the ovaries, early pregnancy or a pregnancy that is not in the uterus i.e., an ectopic pregnancy.
Does it hurt?
Most women do not find this painful but some discomfort is not unusual.
How many times should a pregnant woman get a sonography done during her pregnancy? When should she get it done?
Ideally, a minimum of three scans should be performed during the course of a pregnancy. However, this is subject to availability of resources and economical considerations.
The first scan is usually in early pregnancy, around 6 - 9 weeks, to confirm the location of the pregnancy, to find out whether it is a singleton/ multiple pregnancy, to confirm the presence of fetal heart activity and to confirm the age of the pregnancy to allocate an estimated due date. This is called the 'early pregnancy' or the 'dating' or the 'viability' scan.
The next scan should be performed between 11 - 14 weeks as by this time the baby is fully formed with a head, a body, two legs and two hands. If there are any major structural problems like non-development on the skull, limbs etc., these can be detected. This is also a very important scan to assess the risk of chromosomal abnormalities like Down's Syndrome, in which mental retardation is a major component. This is called the 'nuchal scan'.
The third scan is performed at 18 - 23 weeks. By this time, the baby is much bigger and structures like the heart, kidneys, brain, facial features etc. are better developed and van be well visualised. The aim of this scan is to ensure normality of all these structures, diagnose major and minor problems and arrange further investigations, counselling or follow-ups accordingly. This is called the 'routine anomaly scan'.
The above three scans are the minimum recommended in a 'low-risk' mother in a pregnancy. Further scans, either in-between or later, will depend on the presence of any other problems either in a previous pregnancy, or in the present one. Some Obstetricians routinely request a 'growth scan' later in the pregnancy to assess the growth of the baby, placement of the placenta and the general well being of the baby.
There is no 'maximum' number of scans that can be performed in a pregnancy. The requirement of the scan should be decided based on the clinical scenario.