Pregnancy Ultrasound Scans: An Overview

Pregnancy Ultrasound Scans: An Overview


By Dr. Seema Jain, Gynaecology

An ultrasound test transmits sound waves of very high frequency through the abdomen via a transducer (a device used to get a view of the interior of the abdomen). With the prenatal ultrasound, the resonance is recorded and then transformed into photographic images or video of the baby within the womb.

Ultrasound scan is used during pregnancy to check for:

  1. Presence of multiple fetus
  2. The gestational age or due date
  3. Health of the baby
  4. Location of the placenta
  5. Quantity of amniotic fluids present around the baby
  6. Position of the baby to decide whether to go for Caesarean section or vaginal delivery
  7. Expected weight of the baby

When to go for pre-natal scans?

Ideally, a minimum of three scans should be performed during the course of a pregnancy.

  1. 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.
  2. 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’.
  3. 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. 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.

How is an ultrasound done?

Most ultrasounds that are done after 10 weeks of pregnancy are performed abdominally. The sonographer will place some gel on your belly, and will rub a hand-held device (probe) across your belly in order to obtain a picture of your unborn baby. For an abdominal ultrasound during your first trimester, you may need to drink a few glasses of water. This is so your full bladder will push your uterus up out of your pelvis, allowing the baby to be seen clearly in the ultrasound images.

Sometimes the sonographer needs to perform a vaginal ultrasound. The probe used for this does not need to go in very deep, so most women find that it fits comfortably inside the vagina. This might be recommended if:

  1. you are less than 8 weeks pregnant
  2. you are overweight
  3. the baby is deep inside your abdomen