Ultrasonographic foetal biometry has proven to be a reliable tool in the correct estimation of gestational age and assessment of foetal growth. The choice of a reference chart is critical to the proper assessment of foetal biometry due to observed racial differences. Therefore this study was designed to establish foetal biometric standards in Ghanaians. A prospective, cross-sectional study was conducted using a total of 374 pregnant women with known last menstrual period from the Sunyani Municipal Hospital and the Suntreso Government Hospital from October 2015 to March 2016. Measurements of crown-rump length, biparietal diameter, head circumference, abdominal circumference and femur length were obtained via transabdominal sonography. Results of the present study provide for the first time detailed baseline data on foetal biometry in Ghana and show that there is significant disparity between gestational age estimated by the last menstrual period and ultrasound. Head circumference was the best parameter in estimating gestational age in the second and third trimesters of pregnancy with coefficient of determination (R2) of 96.6% and 84.1% respectively. Combinations of head circumference or biparietal diameter, abdominal circumference and femur length in the third trimester increased the R2 to 90 or 90.5%. Biparietal diameter was a good predictor of gestational age in the third trimester than previously reported in the literature suggesting normal cephalic indices in the present population. Statistically significant differences in foetal biometry exist between the present population and the American, British and Chinese populations in the literature. This study provides preliminary baseline data for the estimation of gestational age and assessment of foetal growth by sonographers and obstetricians.

The estimation of gestational age and the assessment of size and growth of the embryo or foetus are routinely performed during antenatal care (Butt and Lim, 2014). Gestational age is the age of pregnancy (Kalish and Chervenak, 2005). Knowing how long the embryo or foetus is in utero is important in predicting the expected date of delivery and hence proper classification of term, preterm and postterm dates (Blondel et al., 2002). Determination of antimalarial drug regime (Rijken et al., 2012) and the scheduling of initiation date for Zidovudine treatment (Traisathit et al., 2006) depend on knowledge of the gestational age. Accurate dating is also paramount to the proper timing of foetal genetic screening (nuchal translucency, chorionic villi sampling and amniocentesis), testing for foetal lung maturity as well as relating the various maternal blood serum (pregnancy-associated plasma protein-A, alpha-fetoprotein, human chorionic gonadotropin, estriol and inhibin-A) levels to risk factors (Kalish and Chervenak, 2005; Neufeld et al., 2006). Virtually all important clinical decisions in obstetrics are dependent on accurate estimation of gestational age (Merritt et al., 1992).

Traditionally the gestational age is estimated from the first day of the last menstrual period (LMP) since the day of conception cannot be accurately known (MacGregor and Sabbagha, 2008; Whitworth et al., 2015). The reliability of LMP-based gestational age estimation however depends on the regularity of a woman’s menstrual cycle, accurate recall of LMP, interpretation of bleeding in early pregnancy, lactational amenorrhoea, contraceptives use prior to pregnancy and variations in the timing of ovulation and fertilization (Geirsson, 1991; Nguyen et al., 2000; Salpou et al., 2008). About 11 – 42% of gestational age estimated by LMP are reported as inaccurate (Nguyen et al., 2000; Whitworth et al., 2015). The symphysis-fundal height (SFH) measurement is also used as a proxy in estimating gestational age and assessing growth abnormalities (foetal growth restriction and macrosomia) (Ogbe et al., 2015; Robert et al., 2015). The detection rates of small-for-gestational-age babies using SFH ranges from 56% - 86% (Robert et al., 2015). The SFH measurement is affected by the technique used, the number of clinicians involved, multiple pregnancies, status of the maternal bladder, maternal position, pre-pregnant weight, molar pregnancy, amniotic fluid level, macrosomia and intrauterine growth restriction (Engstrom et al., 1993; Steingrímsdottir et al., 1995).

In recent years foetal biometry through the use of ultrasound has become indispensable tool for the practice of obstetrics. Ultrasonography is safe, non-invasive, accurate and cost-effective than other diagnostic imaging modalities (Rueda et al., 2014). Ultrasonographic foetal biometry is the measurements of various structures of the foetal anatomy (Shehzad et al., 2006). Ultrasonography has proven to be the best method for estimating gestational age and the expected due date (Salomon et al., 2011; Butt and Lim, 2014). This has reduced expensive hospitalization and unnecessary interventions such as induction of labour and tocolytic treatments due to wrongfully assumed foetal growth abnormality, preterm and post-term labour (Pemberton et al., 2010; Brakohiapa et al., 2012). Ultrasonographic foetal biometry is more useful in estimating foetal weight and diagnosing intrauterine growth restriction (IUGR) and macrosomia than abdominal palpation and symphysis-fundal height (Salomon et al., 2011; Butt and Lim, 2014).

There are two approaches in the study of foetal biometry: a cross-sectional study and a longitudinal study (Loughna et al., 2009). In a cross-sectional study, a foetus is measured only once during gestation. It is appropriate for creating foetal size and age charts. Foetal size charts and foetal age charts are not synonymous (Briceño et al., 2013). In foetal size chart, the foetal parameter is plotted as a function of gestational age whereas in foetal age chart, the gestational age is plotted as a function of the foetal parameter (Loughna et al., 2009). Longitudinal study involves serial measurements of the same foetus for at least three times during pregnancy. It is best used in creating foetal growth charts (Loughna et al., 2009).

Various foetal biometric parameters have been sonographically measured for the creation of foetal age, size and growth charts (Shehzad et al., 2006). They include measurements of mean gestational sac diameter, transverse cerebellar diameter, liver length, kidney length, intra/interorbital diameters, clavicular length, humeral length, scapula length, sacral length and nasal bone length (Shehzad et al., 2006; Butt and Lim, 2014). The commonly measured parameters, sometimes referred to as the gold standard of foetal biometric measurements, are the crown-rump length (CRL), biparietal diameter (BPD), head circumference (HC), femur diaphysis length (FL) and abdominal circumference (AC) (Shehzad et al., 2006). Many clinical decisions depend upon accurate and reproducible measurements of foetal biometry and choice of appropriate reference charts.

Maternal mortality is unacceptably high across the developing countries, including Ghana. The maternal mortality ratio in 2015 for Ghana was 319 per 100,000 live births and the neonatal mortality in 2013 was 29 per 1,000 live births (UNICEF, 2015). In order to achieve the United Nations Sustainable Development Goals (SDGs) 3.1 and 3.2 of reducing the global maternal mortality ratio to less than 70 per 100,000 live births and neonatal mortality to at least as low as 12 per 1,000 live births by 2030 (UN, 2015) respectively, the role of obstetric ultrasonography cannot be underestimated. An appropriate reference table for ultrasound dating and a reliable reference foetal size chart can improve obstetric management in pregnancy and hence reduce perinatal mortality and morbidity (Lausman et al., 2013; Pay et al., 2015)

Ultrasonographic foetal biometry assumes that the size of an embryo or a foetus is consistent with its age (Butt and Lim, 2014). Different embryos or foetuses of the same biometric measurements can have different gestational ages. Alternatively, different embryos or foetuses of the same gestational age can have the same or different biometric measurements. This indeed is a clinical dilemma since this may suggest normal foetal growth, intrauterine growth restriction (small-for-gestational-age) or macrosomia (large-for-gestational-age). The state of the embryo or foetus can be ascertained by comparing the size or age as the case may be with a reference chart of a specific population derived from low-risk pregnancies. Several reference age and size charts of foetal biometric parameters have been published for populations in Europe (Chitty et al., 1994 a, b, c; Snijders and Nicolaides, 1994; Kurmanavicius et al., 1999 a, b; Paladini et al., 2005; Salomon et al., 2006), America (Deter et al. 1982; Hadlock et al., 1982 a, b, c, d), Asia (Lachman and Shen, 1996; Salomon et al., 2006; Jung et al., 2007) and Africa (Okonofua et al., 1988; Salpou et al., 2008; Mador et al., 2011). These tables and equations have been included in most ultrasound software programme for obstetric use. Since the charts and tables are many, the choice of a reference chart is important in the assessment of foetal biometry (Salomon et al., 2005). An inappropriate reference chart can pose significant clinical implications since this may mislead the obstetrician as to the true state of health or development of the foetus. Using Z-scores, Salomon et al. (2005) observed that the number of foetuses that would have been considered abnormal (below 5th centile and above 95th centile) based on the references by Snijders and Nicolaides (1994), Chitty et al. (1994a, b, c) and Kurmanavicius et al. (1999a, b) to the French population ranged from 2.6% to 23.6% for BPD, HC and FL. The specificity and sensitivity ranged from 90.1% to 99.7% and 39.6% to 67.1% respectively. None of the references for AC was found to be acceptable to the French population.

Differences in foetal biometry have been attributed to race or ethnicity, maternal age, parity, nutritional status and foetal sex (Davis et al., 1993; Jacquemyn et al., 2000; Leung et al., 2008). Even within a population, Krampl et al. (2000) found that geographical changes such as altitude affect foetal size. These have prompted many researchers (Hadlock et al., 1982a, b, c, d, e; Chitty et al., 1994a, b,c; Leung et al., 2008; Westerway et al., 2000; Buscicchio et al., 2008) to develop reference charts that are specific to their populations. Hadlock and coworkers’ charts (Hadlock et al., 1982a, b, c, d, e; Hadlock et al., 1984a) that are commonly used in Ghana were developed over 30 years ago from middle class Caucasian women, which are not representative of the Ghanaian population. Also, reference charts over 30 years old were developed using obsolete ultrasound equipment, suboptimal study designs and statistical analyses (Altman and Chitty, 1994). Some researchers (Westerway et al., 2000; Buscicchio et al., 2008) have even called for the revision of older foetal nomograms due to increasing in birthweights in the last decades (Irgens, 2000).

Cross-sectional reference foetal charts and equations from the Ghanaian population using appropriate methods have not previously been published in the literature. Also, there are no standard reference tables and charts for foetal size and foetal age assessment and a number of ultrasound centres are unable to clarify the reference charts and tables they are using. The choice of reference charts have often being based on preference or on the chart that is loaded by default in the software of the ultrasound machine. The present study attempts to establish reference baseline charts and tables in the Ghanaian population for standard foetal biometric parameters using the methods recommended by Altman and Chitty (1994) and Royston and Wright (1998).

1.2.1 AIM 
To establish the need for foetal biometric standards for Ghanaians.

* To determine menstrual-based gestational age and ultrasound-based gestational age.

* To determine the degree of discrepancy between ultrasound-based gestational age and menstrual-based gestational age.

* To establish reference charts for foetal age and size estimation based on sonographic measurements of crown-rump length, biparietal diameter, head circumference, abdominal circumference, femur length and their ratios.

* To compare the foetal size and age charts in the present study with published charts.

* To determine the best parameter (s) in estimating gestational age in the second and third trimesters of pregnancy.

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Item Type: Ghanaian Topic  |  Size: 197 pages  |  Chapters: 1-5
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