INTRODUCTION

The chest is an integral and vital part of the developing fetus.  Although the fetal lungs are not involved in gas exchange in-utero (as this function is carried out through the placenta via the umbilical cord),¹ a well-developed lung is essential for normal extra-uterine survival.²

Fetuses younger than 24 weeks of gestation are often deemed non-viable due to pulmonary immaturity.²

By 16 – 20 weeks gestation, the normal number of bronchi has formed.³ It has also been established that the fetal thorax grows at a regular rate from 16 to 40 weeks, resulting in a linear correlation between gestational age and thoracic size.⁴ In a normal pregnancy, the ratio between the chest circumference, abdominal circumference, head circumference and femur length also remains constant, with a high correlation coefficient.⁵ Hence, alteration of fetal thoracic dimensions, including chest circumference, may suggest intrathoracic pathologies which may affect the survival of the fetus postpartum.

Along with anomalies of the central nervous and skeletal systems, anomalies of the thorax constitute a major group of fetal abnormalities that are detectable prenatally.² Many of these abnormalities are potentially life threatening, either at the time they are observed, or by virtue of their potential to progress in severity. These intrathoracic abnormalities include congenital diaphragmatic hernia, congenital pulmonary airway malformation (formerly known as cystic adenomatoid malformation), fetal hydrothorax, congenital pulmonary sequestration, neurenteric and bronchogenic cysts, and pulmonary hypoplasia/agenesis.^6–11 Prior to the advent of non-ionizing imaging modalities such as ultrasound and magnetic resonance imaging (MRI), pathologies of the fetal thorax were diagnosed postnatally using radiography with its attendant ionizing irradiation and sometimes misguided management and fatal outcome.¹²

Although MRI is superior to ultrasound in making diagnosis and planning management of fetal chest pathologies,^13,14 the two modalities are complimentary. Overall, ultrasound remains the predominant modality for evaluating disorders related to the fetus and pregnancy largely due to its real-time nature and ready availability compared to MRI which is expensive and not readily available, especially in developing countries.^15–20

Early in gestation, the pulmonary parenchymal appears similar to or slightly less echogenic than the liver. As gestation progresses, there is a trend towards increased homogenous pulmonary echogenicity compared to the liver.² The heart is usually no larger than one-third of the area of the chest and is normally deviated by about 45±20° towards the left side of the fetus.²¹

The degree of lung development is the prime determinant of fetal viability.² Adequate size and shape of fetal thorax, amongst other factors such as adequate amniotic fluid and fetal breathing, have also been noted to influence normal fetal lung growth.  Pulmonary hypoplasia can occur when any of these factors are abnormal.²²

Studies on the sonographic evaluation of the fetal thorax/chest in Nigerians are sparse. The aim of this study was to generate normative references of the fetal chest circumference (CC) in normal late second and third trimester pregnancies in a Nigerian population.

MATERIALS AND METHODS

Study Design: This was a descriptive cross-sectional study carried out from March 2012 to April 2013 on pregnant women with normal singleton fetuses (at 22 – 38 weeks gestation).

Study Setting: The study was conducted at the radiology department of the University of Benin Teaching Hospital, Benin City, Edo state, South-south. Nigeria.

Study Population: These were pregnant women at 22 – 38 weeks gestation referred to the radiology department of the institution. Four hundred and forty pregnant women at fetal gestational age of 22 – 38 weeks were consecutively enrolled having met the following eligibility criteria.

Eligibility Criteria: Inclusion criteria were regular menstrual cycle; known last menstrual period (LMP); and an early dating scan in the first trimester (< 10 weeks), normal live singleton gestation, and intact fetal membranes. The exclusion criteria were unsure LMP, multiple gestation, polyhydramnios or oligohydramnios, premature rupture of membranes, presence of congenital anomalies, maternal medical conditions (gestational diabetes, hypertensive disease of pregnancy, diabetes mellitus, systemic hypertension), and suspected fetal cardiac deviation from the left hemithorax (which may be due to an intrathoracic mass).

Data Collection: After a concise explanation of the study objectives and an assurance of strict confidentiality, written informed consent was obtained from each participant. A detailed medical history was obtained from each subject and documented.  The fetal gestational age was determined by calculation from the last normal menstrual period and validation with an early first trimester ultrasound scan (< 10 weeks).

All the participants were scanned on a Medison Sonoace X4 scanner (Medison Co. Ltd, Gangdong-gu, Seoul, South Korea) with a 3.5 MHz frequency curvilinear transducer. Each participant laid supine on the examination couch with adequate exposure of the abdomen from the level of the xiphisternum to the public symphysis.  The ultrasound coupling gel was applied on the abdomen.  The transducer was then placed on the abdomen and scanning done in different planes with minimal transducer force.  At the level of the four-chamber view of the fetal heart, the transducer was orientated transversely. After ensuring adequate visualisation of the four chambers of the fetal heart, both fetal lungs and ribs on this axial plane, the image was frozen. Tracing around the fetal chest was drawn, with the aid of the electronic calliper, using the ellipse function in order to obtain the fetal chest circumference (Fig. 1). The biparietal diameter (BPD) measurement was obtained from a trans-axial plane of the fetal head at the level of the thalami and cavum septum pellucidum as described by Kurtz et al.²³ The abdominal circumference was measured using the umbilical vein and the fetal stomach as landmarks.²⁴ The femur length was measured from the greater trochanter to the distal metaphysis of the fetal femur.²⁵ Each measurement was taken three times and the average value was recorded to minimize intraobserver error.  All the subjects were scanned by one sonologist so as to reduce interobserver variation.