(See Appendix 10 – Guidelines for the Performance of the Third Trimester Scan)
This scan is not a routine examination and is only indicated for specific clinical reasons. The purpose is to assess fetal size/growth and wellbeing in “high risk” pregnancy to enable management decisions regarding delivery of the fetus/fetuses. Growth restricted fetuses are most reliably diagnosed with ultrasound.
There is evidence from Cochrane reviews that umbilical artery Doppler assessment in high risk pregnancy is associated with a reduction in perinatal mortality. This is of particular relevance to growth restricted fetuses and hypertensive mothers. Monitoring of amniotic fluid volume and biophysical profile assessment in the absence of growth restriction is not supported by high level evidence but may be useful in specific clinical situations e.g. post-dates and diabetic pregnancy.
Limitations of the examination need to be discussed with the patient. Technical limitations which may reduce the accuracy of the examination should be considered and recorded including obesity, severe oligohydramnios, late gestation, macrosomia, excessive fetal movement and breathing.
Monitoring multiple pregnancy is a specific indication for fetal growth assessment. Monochorionic twin gestation requires monitoring prior to 28 weeks for assessment of amniotic fluid volume to diagnose TTTS and both monochorionic and dichorionic multiples require monitoring for growth restriction which may be selective.
The ReferralThe referral should include any reports or images from any previous scans for the current pregnancy and indicate:
- Indication for examination;
- Estimation of gestation and basis eg. menstrual dates, dating scan or Artificial Reproductive Technology;
- History of complications in previous pregnancies;
- Complications in present pregnancy; and
- Chorionicity if known multiple gestation.
The Ultrasound Examination should include the following:
- Fetal biometry including BPD, HC, AC and FL and an estimation of fetal weight derived from these parameters;
- Fetal heart activity and movement;
- Amniotic fluid assessment (cm) usually as an AFI or deepest vertical pool excluding cord which may require colour Doppler identification;
- Umbilical artery Doppler spectral analysis with quantification of waveform by either S/D, PI or RI. Qualitative assessment of the presence or absence of end diastolic flow;
- Fetal movements;
- Limited anatomical survey which may include skull, intracranial anatomy, face, 4-chamber heart with outflow tracts, stomach, bladder and kidneys, abdominal wall, 3-vessel cord, diaphragm, spine and all four extremities long bones with hands and feet. This may be limited by fetal position;
- Placental site and appearance, and where applicable distance of lower margin from internal os;
- In multiple pregnancy include the assessment of both placentas, interfetal membrane, fetal positions with respect to each other and presentation of each, deepest pool of amniotic fluid in each sac. Labelling of Twin 1 and 2 should remain consistent between examinations for purposes of growth assessment; and
- Maternal anatomy including uterus, cervix and adnexal masses.
The Ultrasound ReportThe ultrasound report should include:
- Clinical indication for the scan and the LNMP or EDD;
- Gestational age should be reported as previously derived;
- Fetal presentation;
- Presence of fetal heart activity;
- Biometry (BPD, HC, AC, FL) should be recorded with documentation of centile for each measurement or for estimated fetal weight with respect to gestation. A comment on interval fetal growth where available is desirable;
- Umbilical artery Doppler quantitative measurement and qualitative assessment with interpretation in the clinical context;
- Amniotic fluid volume documented with respect to normal range for gestation;
- Placental localization should be assessed with respect to the relationship of the inferior margin and the internal os and reported as not low-lying or placenta previa. Transvaginal scanning may assist if visualisation is difficult. Description of the position of a placenta previa is clinically useful as is assessment of likelihood of placenta accrete;
- Anatomy visualised and normality or otherwise documented; and
- Multiple pregnancy should be reported with respect to chorionicity and its certainty. The presentation of each fetus and its placenta relative to the uterus, the comparative amounts of fluid in each sac and the relative size of the fetuses should be documented. The interfetal membrane should be identified.