The Debate Over the Importance of the Third Trimester Ultrasound
Should a routine third trimester ultrasound be recommended to healthy pregnant patients? The answer remains the subject of an ongoing debate that has divided countries and healthcare systems for decades.
European proponents note that generations of pregnant individuals have grown accustomed to ultrasound scans at every trimester, particularly where nationalized healthcare facilitates broad access. Yet, the World Health Organization's most recent antenatal recommendations call for only one routine ultrasound before 24 weeks of pregnancy.
As the rift over third trimester ultrasound plays out globally — and more obstetricians routinely offer the exams in late pregnancy — emerging research may help to illuminate a clinical path forward for healthcare providers and their patients.
Debates Surrounding Ultrasound in the Third Trimester
Opponents of routine third trimester ultrasonography bolster their viewpoint with widely cited evidence from a 2015 Cochrane Systematic Review. The review found that routine ultrasound performed after 24 weeks gestation confers little benefit to patients with uncomplicated pregnancies. Notably, the analysis of 34,980 patients from 13 trials points to the lack of improvement in the scan group when compared to the control group in rates of:
- Perinatal mortality
- Preterm birth before 37 weeks
- C-section
- Induction of labor
Yet, as a paper published in BJOG: An International Journal of Obstetrics and Gynaecology points out, much of the research cited in the Cochrane review dates back to the 1970s and 80s, a markedly different era of obstetric imaging. The authors further observe that half of fetal anomalies are diagnosed during the third trimester, including congenital heart defects, which may be missed at an earlier stage of pregnancy.
"In the light of advances in ultrasound technologies and medical knowledge and practices, we can legitimately question the validity of the implementation of current pregnancy management based on the findings of these relatively old studies," the BJOG authors conclude.
Global Differences in Ultrasound Timing
The divide over the value and benefit of a routine third trimester ultrasound has created an international patchwork system based on local guidance, availability, provider training, healthcare coverage and other factors. The U.S., U.K., the Netherlands and Canada draw on the work of the Cochrane review authors as a basis for their obstetric imaging guidance.
For example, the Society of Obstetricians and Gynaecologists of Canada doesn't explicitly outline a third trimester ultrasound protocol, instead advising imaging at 11-14 weeks and at 18-20 weeks to date the pregnancy and assess fetal anatomy in healthy pregnancies. Yet elsewhere, countries such as Belgium, France, Germany and Spain favor routine ultrasound in later pregnancy, pointing to flaws in the Cochrane review and emerging evidence of benefits in the detection of growth problems.
"In developed countries, where advanced ultrasound technology is widely available, there is no reason not to offer this examination routinely to all women," the BJOG authors declare.
US Clinical Recommendations for Ultrasound Timing
The latest practice bulletin from the American College of Obstetricians and Gynecologists (ACOG) and the American Institute of Ultrasound in Medicine (AIUM) deems ultrasound an "important and common" part of obstetric care for all pregnant patients. Where only one screening examination is possible, the guidance suggests timing it between 18-20 weeks of pregnancy. When two screening exams are possible, a first trimester scan, either at 7-10 weeks or at 11-14 weeks in expert settings, can prove valuable in evaluating nuchal translucency, assessing early fetal anatomy and dating the pregnancy.
ACOG and AIUM further add that a third trimester scan may aid in the deeper clinical evaluation of:
- Fetal anatomy
- Fetal growth
- Fetal presentation
- Fetal well-being
- Placental function and health
Additionally, fetal weight may be estimated by measuring the head, abdomen and femur. ACOG, however, cautions healthcare providers to be mindful that weight estimates may be off by up to 20% due to factors such as anatomic parameters, technical issues and patient population.
For patients with clinical symptoms warranting deeper information-gathering, StatPearls reports that third trimester ultrasonography may aid in investigations of:
- Abdominal or pelvic pain
- Amniotic fluid abnormalities
- Differences between calculated gestational date and uterine size
- Ectopic pregnancy
- Fetal demise
- Fetal anomalies or abnormalities
- Hydatidiform mole
- Pelvic mass
- Placental abruption
- Premature rupture of membranes or labor
- Placenta previa
- Uterine abnormalities
- Vaginal bleeding
When fetal growth disturbance is a concern, ACOG and the AIUM consider ultrasound the modality of choice for surveillance. Similarly, practice guidelines from the International Society of Ultrasound in Obstetrics and Gynecology published in 2019 suggest additional scans may aid in monitoring fetal status and detecting fetal growth abnormalities in later stages of pregnancy. Regarding exact timing, the guidance recommends an ultrasound assessment at 36 weeks of pregnancy, a stage deemed more effective than a 32-week ultrasound at detecting fetal growth restriction (FGR) and predicting adverse outcomes to carrying parents and their babies.
Lack of Research Showing Better Outcomes
Some of the ongoing division over routine third trimester ultrasound stems from a lack of research linking later-stage ultrasound surveillance to better outcomes for carrying parents and babies. For example, a 2003 trial among patients administered routine third trimester ultrasounds found a significant reduction in risk of intrauterine growth restriction (IUGR) — yet knowing those risks hasn't translated to fewer complications or neonatal deaths, reports the Taiwanese Journal of Obstetrics and Gynecology.
Seeking to understand why, the authors point to confounding factors, including difficulties distinguishing between cases of a small-for-gestational-age (SGA) fetus, FGR and higher instances of false positives in the exam.
New Areas of Investigation
In terms of other avenues of inquiry for ultrasonography in the last trimester, a promising area of investigation centers on the detection of fetal abnormalities.
Notably, a 2019 paper in Ultrasound in Obstetrics & Gynecology reports that ultrasounds performed between 35–37 weeks of pregnancy uncovered fetal abnormalities that earlier exams hadn't revealed. The prospective study included 52,400 patients in the U.K. undergoing routine ultrasound exams between 2014 and 2019. The most common abnormalities discovered in the late-stage scans included duplex kidney, hydronephrosis, ventricular septal defect, mild ventriculomegaly, ovarian cyst and arachnoid cyst.
The paper builds on a 2011 prospective study in Ultraschall in der Medizin showing that later ultrasound detected 15% of anomalies, suggesting the exam could serve as an important checkpoint in fetal surveillance and proactive management.
The more recent 2021 article in the Taiwanese Journal of Obstetrics and Gynecology adds that third trimester ultrasound also might catch abnormalities that only develop at that stage of pregnancy — including lissencephaly or achondroplasia — permitting healthcare providers to refer their patients for timely appropriate care. Given this reality, the authors advise a renewed push for research to delve further into current and future applications for this modality in the third trimester.
As clinical understanding about evidence-based applications for third-trimester ultrasound grows, healthcare providers may engage their patients in shared decision-making on the appropriate choices for their pregnancies.
Interested in learning more? Watch Guidelines of third trimester OB scan by Dr. A. Sotiriadis
