Reference standards for facial measurements in early third trimester South African fetuses, and the effect of maternal and fetal characteristics

abstract

Fetal alcohol spectrum disorder (FASD) is a major problem worldwide and dysmorphic facial features may be a prenatal biomarker for FASD. Deviations from normal facial development cannot be explored before establishing the normal variation in a specific population, since ethnic differences may exist.

Objectives: Main objective: to establish reference standards for 23 facial measurements on 3D ultrasound volumes obtained between days 196 and 224 of gestation in healthy unexposed South African fetuses from an area with historically high alcohol consumption prevalence and a population group with no existing normative values. Secondary objective: to assess the confounding effect of maternal and fetal characteristics.

Design: This study involves 97 women (including 43 smokers) who had been enrolled in the Safe Passage Study (SPS), a large prospective multinational cohort study assessing the effects of prenatal alcohol exposure. They had adequate 3 D ultrasound volumes of the fetal face acquired at 28⁺⁰–31⁺⁶weeks in singleton pregnancies without comorbidities, congenital abnormalities or exposure to alcohol, marijuana, or methamphetamines from 4 weeks before conception.

Participants, materials, setting, methods: The participants were recruited from two residential areas of low socioeconomic status in Cape Town. Meticulous information was collected on maternal and pregnancy characteristics, including alcohol use at different time points. Gestational age (GA) was based on ultrasound biometry before 24 weeks, and 3D ultrasound volumes were acquired trans-abdominally from a sagittal and axial plane of the fetal face. Volumes were independently assessed offline by two observers and the image with the best landmark definition was used for 23 facial measurements, representing features previously described in children with FASD. The relation to the exact GA was assessed by regression analysis, the expected mean value and standard error of the estimate (SEE) was determined to transform all raw measurements into z-scores, and the effect of possible confounders on z-scores was assessed by ANOVA.

Results: Ten variables changed significantly with advancing GA (extraocular diameter, anteroposterior, medio-lateral and supero-inferior ocular diameter, ocular volume, interlens distance, prenasal thickness, nasal bone length, nose length and nose protrusion) and thirteen did not (interocular distance; interocular: extraocular diameter ratio, prenasal thickness: nasal bone length ratio, pronasal-subnasal distance, subnasal-mouth distance, philtrum length, upper vermillion thickness, nose-philtrum angle, maxillary angle, facial height, facial protrusion, frontomaxillary facial angle and maxilla-nasion-mandible angle). Reference values (expected mean and SEE) for the 23 measurements were established for each day.

The z-scores of all facial measurements were not independently affected by maternal age, parity, gravidity, smoking or body mass index, but infant sex and birthweight z-score significantly influenced several z-scores (infant sex for extraocular, medio-lateral, and supero-inferior ocular diameter, ocular volume, prenasal thickness and nose protrusion; birthweight z-score for extraocular diameter, interocular and interlens distance, nose protrusion and maxillary angle).

Limitations: GA was not always confirmed by first trimester ultrasound and some measurements could not be obtained in all cases due to suboptimal image quality. The cohort included few heavy smokers so an effect of heavy or continued smoking cannot be ruled out, and the effect of ethnicity was not assessed.

Conclusions: These are the first local reference standards for fetal facial measurements and, to our knowledge, the first reference standards for the supero-inferior ocular diameter, face protrusion, upper vermillion thickness, maxillary angle, and nose-philtrum angle. They were broadly in keeping with published references, with small discrepancies explained by minor differences in technique. Even in this narrow GA window, the distribution of many variables changed over time and normal variation was significantly influenced by fetal sex and birthweight z-score. The possible confounding effect of these factors needs to be considered when assessing the impact of harmful exposures like alcohol on facial development.

Keywords:

• Fetal facial measurements
• 3D ultrasound
• confounders

Acknowledgements

We acknowledge Coen Groenewald, local project administrator; Lou Pistorius, for input regarding the methodology, and the PASS (Prenatal Alcohol and Stillbirth and Sudden infant death) network including DM Stat Inc. The funders contributed to the general study design and management of the main and embedded SPS but were not directly involved in the design of this ancillary study, the data collection, the analysis and interpretation of the data, the writing of the manuscript or the decision to submit this manuscript for publication.

Disclosure statement

The authors state that the views expressed in the submitted article are their own and not an official position of the institution or funder. No potential conflict of interest was reported by the author(s).

Author contributions

Lut Geerts: conception and study design, acquisition, analysis, and interpretation of data, original draft, and final version of manuscript.

Rosemary Meyer: study design, acquisition of data, critical review of the final manuscript.

Heidi Nolan: acquisition of data, critical review of the final manuscript.

Lynel Nel: acquisition of data, critical review of the final manuscript.

Daniel Gerhardus Nel: formal statistical analysis of the data, critical review of the final manuscript.

Lucy T Brink: project administration, critical review of the final manuscript.

Hein J Odendaal: conception, acquisition of funding and resources, interpretation of data, critical review of the final manuscript.

Data availability statement

All data on which the conclusions of the paper rely are available to editors, reviewers, and readers DOI 10.25413/sun.15066462 under CCBY 4.0 license.

Additional information

Funding

The study was funded by the National Institute on Alcohol Abuse and Alcoholism (NIAAA), the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), and the National Institute on Deafness and Other Communication Disorders (NIDCD): U01 HD055154, U01 HD045935, U01 HD055155, U01 HD04599 and U01 AA016501.