18-20 Week Screening Pregnancy Ultrasound

Authors:  Dr Monica Pahuja*
                        Dr Michael Bethune*

What are the generally accepted indications for 18-20 week screening pregnancy ultrasound?

• Ultrasound at 18–20 weeks of gestation has become a routine screening procedure in pregnancy, and is ideal for assessing gestational age, foetal number, foetal viability and placental location, and if medically required, foetal gender.
• It is also valuable in detecting abnormalities in amniotic fluid volume.

What are the prerequisites for having an 18-20 week screening pregnancy ultrasound done?

The gestational age should be 17 weeks 6 days to 21 weeks 6 days as the ideal.

What are the absolute contraindications for an 18-20 week screening pregnancy ultrasound?

There are no absolute contraindications.

What are the relative contraindications for an 18-20 week screening pregnancy ultrasound?

There are no relative contraindications.

What are the adverse effects of an 18-20 week screening pregnancy ultrasound?

There are no adverse effects. Medical ultrasound has been in use since 1950. There is no evidence in the medical literature to show any biological or hazardous effects to the foetus. It is a test that has no ionising radiation.

Is there any specific post-procedural care required after 18-20 week screening pregnancy ultrasound?

No specific post-procedural care is required.

Are there alternative imaging tests, interventions or surgical procedures to an 18-20 week screening pregnancy ultrasound?

Foetal magnetic resonance imaging (MRI) might be used to confirm abnormalities seen on ultrasound after 21 weeks gestation. MRI is not a screening examination and most MRI facilities do not offer this service, as it is highly specialised and usually carried out in conjunction with a comprehensive counselling and general foetal diagnostic service.

An obstetrician will usually be guided by a maternal foetal medicine specialist clinic or geneticist regarding the appropriateness of a referral for MRI. MRI is used to clarify abnormalities that are strongly suspected on ultrasound of the foetus, at or after 21 weeks’ gestation if other testing, such as amniocentesis and blood testing, fails to reveal a chromosomal abnormality that would explain the abnormal findings.

Further information about 18-20 week screening pregnancy ultrasound:

There is a reduction in the sensitivity of ultrasound for the detection of foetal abnormalities in obese patients, as the probe is at a greater distance from the foetus.

It might not be possible to see all the foetal structures in multiple pregnancies. Patients might be asked to return on another day to complete the examination when the foetuses might lie in a more optimal scanning position.

*The author has no conflict of interest with this topic.

Page last modified on 29/3/2017.