[CG] Mild ventriculomegaly on antenatal ultrasound


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Fetal ventriculomegaly is a common finding on antenatal ultrasound and is defined as an atrial measurement of ≥ 10mm of the posterior horn of the lateral ventricle (1). It can be further subdivided into mild 10-12mm, moderate 13- 15mm and severe >15mm (2). It has a prevalence of approximately 1% (3). Ventriculomegaly has a range of causes; normal variation, aneuploidy, genetic syndromes, primary brain structural abnormalities, congenital infections, cerebrovascular accidents and intracranial haemorrhage. Prognosis and corresponding counselling of the parents is dependent on the cause of the ventriculomegaly, the antenatal progression and any co-existing abnormalities(4). It is therefore vitally important to look for  any underlying aetiologies and co-existing CNS and non-CNS abnormalities in order to present the parents with the most relevant and accurate information.


Accurate measurement of the ventricles is important in both defining ventriculomegaly and also assessing progression. The fetal head should be scanned in the axial plane at the level of the frontal horns and the cavum septum pellucidum (CSP) (the same level at which a head circumference is taken), at an appropriate magnification that the head fills the screen. The callipers should be placed at the internal margins of the atrial walls at the level  of the parietal occipital groove and the glomus of the choroid plexus, perpendicular to the axis of the ventricle.

Although the distal ventricle is always easier to see than the proximal one because of reflection of the ultrasound beams from the fetal skull, both ventricles should be checked; ventriculomegaly is unilateral in 50-60% cases and bilateral in 40-50% (5).


Once ventriculomegaly has been diagnosed, there should be a detailed, sonographic evaluation of the neuroanatomy by a medical sonographer. Whether this is by transabdominal or transvaginal ultrasound will depend on the preference of the patient, the sonographer and the fetal position.

Other, non-CNS structures should also be carefully assessed including fetal biometry looking for evidence of growth restriction, the heart and any markers of intrauterine infection.

Testing for genetic disorders

Parents should be offered invasive, diagnostic testing and chromosomal microarray (CMA). 

Between 0-5% (2, 5) of fetuses with apparently isolated m i l d ventriculomegaly will have an underlying abnormal karyotype, most commonly Trisomy 21 and a further 10-15% will have abnormalities found on CMA.  

Testing for fetal infection

Congenital infections, most commonly cytomegalovirus (CMV), toxoplasmosis, parvovirus and Zika have been associated with mild ventriculomegaly in around 8% of cases (5). Parents should be offered tests for CMV, toxoplasmosis and parvo virus (regardless of history of known exposure or symptoms). Women with mild fetal ventriculomegaly who have been to a Zika area and not yet tested should be offered a test.

Fetal MRI

Fetal MRI (fMRI) can be a useful adjunct to ultrasound if the relevant radiological expertise and technology is available. The additional information will depend on the size of the ventricles as well as the quality of the original ultrasound and the level of expertise in the practitioner. The chance that fMRI will find important, clinically relevant additional brain abnormalities not picked up on ultrasound varies in the literature from 1-14% with the most recent studies putting the figure at 5-6% (5). The most common abnormality picked up on fMRI after being missed on ultrasound is agenesis of the corpus callosum.

Women wishing to have a fetal MRI, to look for additional brain abnormalities that may affect the prognosis, after appropriate counselling should be referred to fetal medicine department for review.

  • The fetal medicine department will arrange the MRI
  • Fetal medicine will review again after MRI to discuss results.
  • Thereafter the patient will go back to their own unit, unless otherwise indicated and delivery will be planned in their own unit.

Follow up antenatal ultrasound examinations

There are no data on optimal timings of follow up assessments once a diagnosis has been made. A suggested pragmatic approach would be 4-weekly assessments. Progression of ventriculomegaly is an important prognostic indicator; evidence suggests that 5% progress during pregnancy (5).


The timing and mode of delivery should be planned as per normal obstetric indications. An alert should be placed on the electronic BadgerNet record to ensure that neonatologists are made aware of the antenatal diagnosis.

Cord bloods should be taken with parental consent for chromosomal analysis and congenital viral infections from those infants who didn’t have antenatal testing.

Postnatal follow up should be arranged by the neonatologists prior to discharge from hospital.


Most of the statistics quoted in the literature are based on whether the ventriculomegaly is apparently isolated or not; true isolation will only be able to be confirmed postnatally. Neurodevelopmental delay in case of isolated unilateral mild or moderate ventriculomegaly is thought to be 6% (5); in bilateral isolated ventriculomegaly this rises to 8-12% (7). This may not be dramatically higher than the background population risk. Long-term prognosis also depends on associated findings and the positive results of any investigations. 

Parents should be offered antenatal counselling by paediatricians to discuss prognosis and postnatal care in greater detail. A patient information leaflet from ISUOG and a link to further information is below.

Further information

Melchiorre, K & Bhide, Amarnath & Gika, Artemis & Pilu, G & Papageorghiou, A.T. (2009). Counseling in isolated mild ventriculomegaly. Ultrasound in Obstetrics & Gynecology

Patient Information:

ISUOG. Ventriculomegaly
"This leaflet is to help you understand what Ventriculomegaly is, what tests you need, and the implication of having been diagnosed with Ventriculomegaly for you, your baby and your family."

  1. International Society of Ultrasound in Obstetrics and Gynecology Education Committee. Sonographic examination of the fetal central nervous system: guidelines for performing the ‘basic examination’ and the ‘fetal neurosonogram’. Ultrasound Obstet Gynecol 2007; 29: 109 – 116.
  2. Society for Maternal-Fetal Medicine (SMFM): Fox NS et al. Mild Fetal Ventriculomegaly: diagnosis, evaluation and management. SMFM Consult Series 45 2018.
  3. Pilu G, Hobbins JC. Sonography of fetal cerebrospinal anomalies. Prenat Diagn 2002; 22: 321 – 330.
  4. Scala C, Familiari A, Pinas A, Papageorghiou T, Bhide A, Thilaganathan B, Khalil A. Perinatal and long-term outcomes in fetuses diagnosed with isolated unilateral ventriculomegaly: systematic review and meta-analysis. Ultrasound Obstet Gynecol 2017; 49: 450–459
  5. Griffiths PD, Brackley K, Bradburn M, et al. Anatomical subgroup analysis of the MERIDIAN cohort: ventriculomegaly. Ultrasound Obstet Gynecol 2017;50:736-44.
  6. RCOG/RCM/PHE/HPS Clinical Guidelines. Zika Virus Infection and Pregnancy. Updated Feb 2019.
  7. Pagani G, Thilaganathan B, Prefumo F. Neurodevelopmental outcome in isolated mild fetal ventriculomegaly: systematic review and meta-analysis. Ultrasound Obstet Gynecol 2014; 44: 254 – 260.

Last reviewed: 01 April 2021

Next review: 01 April 2024

Author(s): Dr Rachel Bradnock

Approved By: GONEC

Document Id: 916