Obstetric Ultrasound

Pregnancy Month 4

Obstetric Ultrasound

Obstetric ultrasound is a test that assists obstetricians in diagnosing and screening pregnant women for any abnormalities in the fetus, as well as anomalies in the mother’s pelvic organs including the uterus and the ovary. It is a safe procedure as it uses sound waves as opposed to radiation.

Types of Ultrasound

2D ultrasound

  • Creates an image without depth to observe fetal size and any abnormalities of organs, as well as to observe the pelvic organs of the mother.

3D ultrasound

  • Creates a virtual still image with depth to be used in conjunction with 2D ultrasound to enhance the effectiveness of diagnosis in case there are abnormalities found in the fetus.

4D ultrasound

  • Creates a 3D moving image with depth of the fetus’s postures. The objectives of using 4D ultrasound are the same as those of 3D.

Remark: The screening test during pregnancy is principally performed by 2D ultrasound. 3D and 4D ultrasounds are used only when the doctor deems them appropriate.

Gestational Age and Indications

Ultrasound can be performed in every gestational age in accordance with the doctor’s opinion. Appropriate gestational ages for ultrasound are as follows:

5-11 Weeks

  • To confirm a intrauterine pregnancy and diagnosis for a multiple pregnancy
  • To determine the gestational age by measuring the size of the fetus
  • To diagnose excessive bleeding beneath the placenta membrane
  • To diagnose an abnormal pregnancy including ectopic pregnancy, blighted ovum, and molar pregnancy
  • To diagnose myoma uteri and ovarian cyst or tumor

11-13 Weeks and 6 Days

  • To measure the size of the fetus and determine the gestational age
  • To measure fluid accumulation beneath the skin of fetal neck (nuchal translucency). The result of the measurement is used in conjunction with the mother’s blood test for down syndrome screening*
  • To identify the early stages of disabilities in the fetus
  • To evaluate the mother’s pelvic organs such as the uterus and the ovary

Remark: If the fetus has unusual thickness of nuchal translucency, it is possibly related to down syndrome or other chromosomal abnormalities. In addition, this is possibly related to fetal abnormalities including heart diseases, abnormal limbs, genetic disorders, etc.

18-23 Weeks

This is a crucial age and takes the longest to perform ultrasound (30 minutes-1 hour)

  • To screen and diagnose structure-related abnormalities such as of the skull, cerebral tissue, spinal cord, arms, legs, heart, etc.
  • To observe fetal growth, the position of the placenta and estimate the amount of amniotic fluid
  • To observe the length of the cervix in order to determine the possibility of a premature birth

28 weeks until the immediate stage before the delivery

  • To observe development and determine the weight of the fetus
  • To diagnose fetal growth restrictions
  • To check the position of the placenta, fetal posture and the amount of amniotic fluid
  • To observe the length of the cervix in order to determine the possibility of premature labor
  • To check the health of the fetus by observing its breathing, movement and evaluating amniotic fluid


  • If the mother has no risk factors, it is recommended that an ultrasound is performed during the 30-32 weeks.
  • If the mother is at potential risk, having a multiple pregnancy or complications, it is recommended that the health of the fetus should be evaluated as often as per the doctor’s suggestion.

The Effectiveness and Limits of Ultrasound

Ultrasound tests need water to create an image and depend on the sound waves penetrating through the tissues. Therefore, the effectiveness of an ultrasound depends on the amount of amniotic fluid present, the thickness of the mother’s abdomen, obesity, surgical wounds at the abdomen or whether or not the fetus is ready for the test.

Recommended Doctor

Thewin Dejthevaporn, M.D. Summary:  Obstetrics And Gynaecology  Ob/gyn, Women - Gynecology