(Also see Appendix 8 – Guidelines for the Midtrimester Obstetric Scan)
This scan is available for all patients as part of routine obstetric management. Cochrane evidence indicates a reduction in induction for post-dates pregnancy, increased rates of termination for fetal malformation and increased diagnosis of multiple pregnancy. There is no current evidence that perinatal mortality rates are reduced by this investigation.
It is not recommended as a screening test for aneuploidy.
Rates of detection for structural fetal abnormality are about 50-60% overall and are about 30% for cardiac abnormalities.
Diagnostic limitations of the examination need to be discussed with the patient. Technical limitations which may reduce diagnostic accuracy at the time of the examination should be recorded e.g. fetal position, maternal obesity and abdominal scars.
A morphology scan may also be performed as a second opinion for confirmation or further investigation of pathology.
The ReferralThe referral should include any reports or images from any previous scans for the current pregnancy and indicate:
- The purpose of examination: routine or further evaluation for suspected abnormality;
- LNMP and/or information on which dates have been derived including infertility management or previous dating ultrasound;
- Past history of fetal abnormalities;
- Chorionicity if known multiple gestation; and
- Potential teratogenesis - epilepsy, diabetes, drugs etc.
The Ultrasound ExaminationThe ultrasound examination should include:
- Fetal number;
- Fetal heart activity;
- Fetal biometry including BPD, HC, AC and FL;
- Anatomical survey (see Appendix 8 - Guidelines for the mid-trimester obstetric scan);
- Fetal movement;
- Amniotic fluid assessment;
- Cervical length;
- Placental site, distance of lower margin from internal os; and
- Maternal anatomy including uterus and adnexal masses.
The Ultrasound ReportThe ultrasound report should include:
- Indication for the scan, and the LNMP or EDD;
- Presence of fetal heart activity;
- Gestational age calculated as a single composite of biometric measurements (usually BPD, HC and FL). Ultrasound estimation of gestation should be used in preference to menstrual dates if the discrepancy is 10 days and there is no previous dating scan or known conception from artificial reproductive technologies. Acknowledgement should be made that the dates have been changed;
- Fetal anatomical survey should be reported and commented on with respect to detection of malformations and quality of assessment. Examinations that are limited or suboptimal because of fetal position, maternal obesity or scars should be acknowledged. A judgment should be made as to whether a repeat examination would improve diagnostic accuracy;
- Fetal abnormalities should be reported to the referring doctor on the same day to expedite management choice for the pregnant woman. Different practices will exist with respect to the sonographers and/or sonologist informing the woman at the time of the scan but written protocols should be developed within each practice.;
- Soft markers of aneuploidy may be detected. Interpretation of the clinical significance of these is controversial. Routine follow up is rarely indicated;
- Maternal anatomy including uterus, cervix and adnexal masses;
- Placental localization should be assessed with respect to the relationship of the inferior margin and the internal os and reported as low-lying or not. Transvaginal scanning may assist if visualisation is difficult. Description of the position of a low-lying placenta is clinically useful. Low-lying placentas (≈5%) should be reviewed in the third trimester with the possibility of a repeat scan;
- Amniotic fluid volume is generally subjectively assessed and reported as normal or otherwise. Four quadrant or deepest pool quantification may be used if levels are abnormal;
- Multiple pregnancy should be reported with respect to chorionicity and its certainty. The location 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; and
- Conclusion with gestational age and anatomy overview stated.