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Obstetric UltrasoundObstetric ultrasound was first introduced in the late 1950s. It is now widely used and has become a useful tool in monitoring anddiagnosis. Ultrasound scans use sound waves which are safe for you and your baby.

Booking scanThe first ultrasound scan is usually performed before 15 weeks. [1] The purpose is to:

Diagnose pregnancy.Accurately determine gestational age - this is essential for intervention of post-maturity, and for accurate serum screening forDown's syndrome.Determine viability - to confirm the presence of a heartbeat and exclude ectopic pregnancy and hydatidiform mole.Determine fetal number and, in multiple pregnancies, the chorionicity/amnionicity.Detect gross fetal abnormalities.

ProcedureMeasurement of crown-rump length accurately measures gestational age if performed before 13 weeks. After 13 weeks, thefetus becomes increasingly flexed so results are inaccurate. Alternatives that can be used after this include bi-parietaldiameter, and/or head circumference, or femur length.it is usually performed abdominally, although occasionally a vaginal scan is necessary.Nuchal translucency scans for risk of Down's syndrome are best performed between 10-14 weeks.

Screening for structural abnormalities - anomaly scanThis scan is offered to pregnant women ideally between 18-20 weeks of gestation. [1] This scan can provide dating information anddiagnosis of multiple pregnancy, in units where no booking scan is performed.

The main purpose is:

To reassure the mother that her baby appears to have no gross structural abnormalities:50% of significant abnormalities will be detected by the 20-week screening scan.To provide the parents with options - eg, termination, preparation, and appropriate care throughout the rest of thepregnancy and delivery.

To determine placental morphology and localisation:Where the placenta extends across the internal cervical os, another scan at 36 weeks should be offered.

To confirm that fetal growth is appropriate.

ProcedureAssess growth by the measurement methods below:

Bi-parietal diameter (most accurate for dating up to 20 weeks)Head circumferenceFemur lengthAbdominal circumference

Look at the head shape and internal structures:Cavum septum pellucidumCerebellumVentricular size at atrium (<10 mm)

Minimum standards:Spine: longitudinal and transverseAbdominal shape and content at the level of the stomachAbdominal shape and content at the level of the kidneys and umbilicusRenal pelvis (<5 mm AP measurement)Longitudinal axis - abdominal-thoracic appearance (diaphragm/bladder)Thorax at level of four-chamber cardiac viewAortic archArms - three bones and hand (not counting the fingers)Legs - three bones and orientation of feet (not counting the toes)

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Optimal standards:Cardiac outflow tractsFace, nose and lips; 15% of women may have to return for further checks

Aneuploidy checksAneuploidy scans are not routinely performed, as many normal pregnancies may have some of these features - ie there is a high false-positive rate. [1] Pregnancies affected by aneuploidy (abnormal chromosome number) will have sonographic markers. However, 50-80%of affected cases will already be identified by triple test, maternal age and nuchal translucency measurements.

Indications for a 'marker' scan include:

Family history of abnormalities, such as a neural tube defect (NTD)Multiple pregnanciesMaternal diabetes or epilepsyRecurrent miscarriagePolyhydramniosAlpha-fetoprotein (AFP) abnormal/maternal age >35 yearsOligohydramnios

Ultrasound checklist for screening for aneuploidy

Common sonographic 'markers' for aneuploidy Other risk factors

Choroid plexus cyst Maternal age

Ventriculomegaly (>10 mm at the atrium) Serum screening results

Echogenic bowel (equivalent to bone density) Nuchal translucency (10- to 14-week scan)

Head shape

Nuchal pad (>5 mm at 20 weeks)

Cysterna magna

Cleft lip

Echogenic foci in heart

Dilated renal pelvis (>5 mm AP)

Short femur/humerus

Talipes

Sandal gap

Clinodactyly

Clenched hand

Two-vessel cord

General standardsThese are set by the Royal College of Obstetricians and Gynaecologists to assure the quality of service provision. They includeproviding clear, written advice that includes detection rates for defined, common conditions. A trained counsellor in the area of diagnosisand screening should be available, as should a quiet room for breaking bad news about the baby. It should be possible to discuss thefindings with an obstetrician within 24 hours or soon after detection of the anomaly.

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Potential detection rates based on Royal College of Obstetrics and Gynaecologyscreening strategyUsing the standard 20-week scan checklist:

Anencephaly - 100%Spina bifida - 92%Major cardiac anomalies (hypoplastic ventricle) - 61%Diaphragmatic hernia - 62%Gastroschisis - 100%Exomphalos - 92%Major renal tract problems (renal agenesis) - 85%

Fetal presentation and cervical lengthSuspected fetal malpresentation (eg, breech) should be confirmed by an ultrasound examination after 36 weeks. [2]

Cervical length measured by transvaginal ultrasonography in asymptomatic high-risk women predicts spontaneous pretermbirth at less than 35 weeks of gestation. [3]

Biophysical profile screeningThis is a type of fetal assessment using specific criteria to reach a well-being score for high-risk pregnancies. It is based on the Apgarscoring system used to assess the condition of the newborn.

It was introduced in the 1980s and, despite minor refinements of the original test, the assessments still include five main features:

Monitoring of fetal movementsFetal toneFetal breathingAssessment of amniotic fluid volumeAssessment of fetal heart rate by electronic monitoring

Biophysical profile fetal assessment is based on the association between low biophysical scores and poor pregnancyoutcome. [4]Hence, the procedure aims to detect acute and chronic fetal compromise with changes in fetal heart patterns, decreasedbody and breathing movements, reduced amniotic fluid, oliguria, and redistribution of regional blood flow leading to a reduction in fetalrenal blood flow.

Doppler ultrasoundThe usual ultrasound scans in pregnancy use sound waves which are safe for you and your baby but cannot show blood flow. Dopplerultrasound uses high-intensity sound waves to detect the blood circulation in the baby, uterus and placenta.

The application has extended from the umbilical cord to fetal vessels (aorta, cerebral and renal arteries) as well as maternalvessels supplying the placental intervillous space.It is used for high-risk pregnancies where there is concern about baby's well-being - eg, intrauterine growth restriction,hypertensive disorders of pregnancy - and to distinguish between the normal small fetus and the 'sick' small fetus.Despite its advances, Doppler ultrasound is not of use in routine antenatal screening because several studies have shown itis an unnecessary intervention and may cause possible adverse effects. [5] Its current role in optimising management,particularly timing of delivery, remains unclear.

Further reading & referencesWoo JSK; Obstetric Ultarsound, a Comprehensive Guide

1. Ultrasound screening; Royal College of Obstetricans and Gynaecologists, 20002. Antenatal care for uncomplicated pregnancies; NICE Clinical Guideline (March 2008, updated 2017)3. Crane JM, Hutchens D; Transvaginal sonographic measurement of cervical length to predict preterm birth in asymptomatic women at increased risk:

a systematic review. Ultrasound Obstet Gynecol. 2008 May;31(5):579-87. doi: 10.1002/uog.5323.4. Lalor JG, Fawole B, Alfirevic Z, et al; Biophysical profile for fetal assessment in high risk pregnancies. Cochrane Database Syst Rev. 2008 Jan 23;

(1):CD000038. doi: 10.1002/14651858.CD000038.pub2.5. Westergaard HB, Langhoff-Roos J, Lingman G, et al; A critical appraisal of the use of umbilical artery Doppler ultrasound in high-risk pregnancies:

use of meta-analyses in evidence-based obstetrics. Ultrasound Obstet Gynecol. 2001 Jun;17(6):466-76.

Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. PatientPlatform Limited has used all reasonable care in compiling the information but makes no warranty as to its accuracy. Consult a doctoror other healthcare professional for diagnosis and treatment of medical conditions. For details see our conditions.

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Author:Dr Colin Tidy

Peer Reviewer:Prof Cathy Jackson

Document ID:2532 (v23)

Last Checked:06/09/2013

Next Review:05/09/2018

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