Health Science Journal

Introduction

Ponderal Index (PI) is used to assess “proportionality” in small-for-gestational age (SGA) infants since it is a weightheight related parameter [1]. The SGA newborns constitute a special group of neonates who may have suffered intrauterine insults and deprivation [2]. Early intrauterine adverse insults resulting to foetal growth restriction or genetic disorders often occurred with proportionate small foetus [1,3]. Neonates have been classified into SGA, appropriate for gestational age (AGA) and large-for-gestational age [4]. High morbidity and mortality perinatal rate are normally associated with foetuses with growth restrictions [5].

Foetal leanness has been assessed by foetal ponderal index (FPI) which determine pattern of growth and predicts pregnancy outcome [3,6]. Foetal ponderal index was defined as the inverse ratio of estimated foetal weight in grams to the third power of the femur length [6]. This means that the FPI is directly proportional to estimated foetal weight and inversely proportional to the third power of femur length. Foetal malnutrition which is evidence in foetal wasting is a condition mainly due to poor health and malnutrition of mother during pregnancy [3,7]. Geographical location, racial differences, ethnicity and socioeconomic status could have effect on foetal anthropometry; therefore each locality should develop its own standard of PI values [4]. The placenta is established early in intrauterine life. Its rapid growth in the early part of pregnancy is essential for the supply of the nutrients required to ensure adequate foetal growth through metabolic endocrine functions [8]. Foetal growth is largely measured by the availability of nutrients from the mother as well as placenta capability to supply these nutrients in proper quantities to the foetus [8]. A linear increase of the placenta size correlates with gestational age all through pregnancy [8]. Placenta size can help to determine pregnancy outcome [9]. Large placenta is indicative of maternal diabetes mellitus whilst placenta less than 2.5 cm thick is generally associated with intrauterine growth restriction [9,10]. Mothers of SGA babies were found to have significantly low anthropometric characteristics than mothers of normal and LGA babies, including weight, height and BMI [2,11]. Reduction in maternal nutrition during pregnancy has a direct relationship with the maternal BMI and may have adverse effect on the placenta and hence could impair intrauterine growth [8]. Maternal nutritional status has an important effect on foetal size and birth weight [12]. Foetal growth by sonographic measurement is directly related to maternal anthropometry and placenta volume in pregnancy and is mediated through maternal weight and weight gain [8]. The values of FPI have been assessed by the use of ultrasound [3,6]. Studies suggest associations between PV, maternal anthropometry, gestational age, foetal growth and birth outcome [8,13,14]. Literatures have shown that PI is prognostic factors in the evaluation of intrauterine growth restriction [2-4,15]. Association between placenta thickness and estimated foetal weight has been demonstrated by researchers [16,17]. This study intends to determine normal value of FPI according to gestation and its relationships with maternal anthropometry and placenta volume in the second and third trimesters. It is envisaged that established values of FPI would be useful in making assessment of intrauterine growth restriction in singleton foetus. Known value of FPI in relation to placenta volume and maternal BMI at a particular gestation would help clinician to have an understanding of foetus at risk and profile necessary dietary intake to expectant mother.

Materials and Methods

A total of 384 apparently health pregnant women at the time of this study, aged between 18 and 34 years in the second and third trimesters were prospectively enrolled into the study at the Radio diagnostic Department of City Gate Health Diagnostic Services, Ogijo Ogun State from June 2017 to September 2017. The study was approved by the Nigerian Institute for Medical Research Institutional Review Board (NIMR-IRB). Written informed consent was obtained from all the subjects after they were fully instructed about the investigation. Gestational ages estimated by using foetal biometry such as bi-parietal diameter (BPD), femur length (FL) and abdominal circumference (AC). Besides smokers and alcohol addicted mothers other exclusion criteria were abnormal blood pressure (BP), diabetes mellitus, history of IUGR, congenital malformations and multiple gestations. Each subject was scanned once, at the second and third trimesters (14-28 weeks and 29-40 weeks of gestation respectively), the maternal weight, height, body mass index (BMI), Blood sugar, blood pressure and data of ultrasound examination were recorded. All sonographic examinations were performed transabdominally using a Siemens real-time scanner (SL 1 Sonoline Siemen) with a 3.5 MHz sector and 5 MHz linear transducers. The placenta volume (PV) or estimated placenta volume (EPV) was measured in three dimensions (height, thickness and width) by placing the transducer perpendicular to the placenta plane and sweeping the transducer to achieve placenta full length as much as possible. In order to exclude inter and intra observer variation [18], the sonographic measurement was taken by only one experienced sonographer. The foetal ponderal index (FPI) was calculated as follow:

FPI=EFW/FL

Where EFW is estimated foetal weight in g derived from the measurement of the AC, FL, BPD and HC. The FL is the foetal femur length in cm. The gestational age was expressed in weeks after sonographic measurement. The FPI was documented in grams per centimeter cube (g/cm3). The PV was expressed in centimeter cube (cm3). The maternal body mass index (BMI) was derived from the ratio of the mothers weight and the height and was measured in kilogram per centimeter square (kg/m2). The gestational age was grouped into second and third trimesters. Data were analyzed in line with the objectives of this study using statistical package for social sciences (SPSS, Chicago, USA) version 20. Sonographic measurements of PV, FL, EFW and derived values (FPI and maternal BMI) at different gestational ages were expressed as mean plus or minus standard deviation. Pearson’s correlation analysis was used to establish the association between FPI and PV as well as relationship between FPI and maternal BMI. Pvalue of less than 0.05 was considered statistically significant.

Results

Table 1 showed both maternal and foetal variables in the second and third trimesters. The mean age of the studied subjects was 27.3 4.4 years. The mean maternal weight, height and BMI were 61.1 5.8 kg, 1.6 0.1 m and 23.5 2.3 kg/m2. The mean values of FPI, PV and maternal BMI according to gestational age were shown in Table 2. Table 3 illustrated the mean value of FPI, PV and maternal BMI in the second and third trimesters separately. There was significant correlations between FPI and PV in the second trimester; r=-0.680, p<0.05 and in the third trimester; r=-0.410, p<0.05. Significant correlation was also noted between FPI and maternal BMI in the second trimester; r=0.156, p<0.05 and in the third trimester; r=-0.268, p<0.05. Figures 1 and 2 show a fairly negative linear relationship between FPI and PV in the second and third trimesters. Figures 3 and 4 showed both weakly positive and negative linear relationship between FPI and maternal BMI in second and third trimesters respectively.

Table 1 Descriptive statistics of measured maternal and foetal variables.

PV: Estimated Placenta Volume, EFW: Estimated Foetal Weight, AC: Abdominal Circumference

Table 2 Descriptive statistics of Foetal Ponderal Index, placenta volume and maternal body index throughout gestation studied.

Table 3 Comparison analysis of variables studied in second and third trimester.

Discussion

Foetal Ponderal Index (FPI) was determined by ultrasound which has been possible in previous studies [3,6]. This study provides foetal ponderal index nomogram for normal pregnancies in the second and third trimesters. Foetal ponderal index in this study gradually decreases in the second trimester as the gestational age advances, with the same pattern maintained in the third trimester up to term. Similarly, the studies of Oluwafemi et al. [2] and Rashidi et al. [7] done on neonates 24 hours after delivery showed that PI decreases as gestational age advances up to 42 weeks gestational age in the third trimester. This present study revealed that the range of FPI in the second trimester is between 13 g/cm3 and 10 g/cm3 and in the third trimester is between 9 g/cm3 and 8 g/cm3. Study done by Vilbergsson et al. [6] revealed that this range of PI is possible in-utero. The FPI values obtained in this study for the different gestational age in the second and third trimester are however not precisely the same with the previous studies of Vilgerbsson et al. [6] and Thame [12]. The absolute standards of the FPI of previous studies were different from each other too, when the studies were compared for a given gestational age. This variability could have arisen due to environmental location of the subjects, race and possibly socio economic factors. Studies carried out by Morris et al. [19] and Rondo [20] in Brazilian was in agreement with the result of the present study. Studies carried out by Haggarty et al. [21] and Onyiriuka et al. [22] reported lower values of PI than the normal range as the gestational age increased to term (≥40 weeks). This value was possible because of expectant mothers’ ill-health conditions. The present study is in keeping with the previous studies conducted by Roje et al. [4], Oluwafemi et al. [2] and Rashidi et al. [7] which showed that FPI standards have to be defined for specific population in order to eliminate variations resulting from geographical locations, race and socioeconomic groups. The range of placenta volume across gestational age was 42.60 cm3 to 620.40 cm3 which is similar to results obtained in some previous studies [8,12]. This study and previous studies [8,12] showed that placenta volume increases gradually as the gestational age increased in the second and third trimesters. The method adopted for the estimation of placenta volume in this present study is in line with the method used by Azpurua et al. [23]. This method is simple, rapid and accurate. It makes it practical for routine prenatal care. Significant fairly strong correlations were found between the foetal ponderal index and estimated placenta volume in both the second and third trimesters. This was in agreement with most of the previous studies of Thame et al. [8], Thame [12] and Hasegawa et al. [24], which reported that FPI is correlated with placenta volume in the second and third trimesters. The present study shows no information about the first trimester foetuses. Although, some studies [8,12,25] conducted in the first trimester reported on placenta volume, they did not give any information on its correlation with FPI, but reported in their studies that PV may influence FPI. Thame [12] opined that PV was shown to be an earlier predictor of FPI in his study to investigate maternal nutrition and its effect on birth outcome. Significant weak correlations were found between the FPI and maternal BMI in the second and third trimesters. Most previous studies appreciated the fact that maternal BMI is influenced by weight gained in pregnancy which affect foetal outcome [4,8,12]. An earlier study by Haggarty et al. [21] disagrees with subsequent studies of Roje et al. [4], Thame et al. [8] and Thame [12] on the influence of FPI in predicting IUGR. Although, studies of Thame et al. [8], Roje et al. [4] and Thame [12] showed good correlation between ponderal index and maternal BMI, this present study confirmed the level of correlation in the second and third trimesters.

Conclusion

Nomogram of foetal ponderal index in the second and third trimesters has been established by this study. Foetuses in the second trimester (14-28 weeks) have mean FPI of 10.78 3.19 g/cm3 and foetuses in the third trimester (29-42 weeks) have FPI of 9.75 1.66 g/cm3. Foetal ponderal index decreases with increase in the gestational age in the second and third trimesters up to term. The FPI has fairly strong negative significant relationship with the PV in the second and third trimesters. The correlation coefficient were (r=-0.680; p=0.000 and r=-0.410; p=0.000) respectively. There was a weak positive significant relationship between FPI and maternal BMI in the second trimester and also weak, but negative significant relationship in the third trimester. The correlation coefficient were (r=0.156; p=0.041 and r=-0.268; p=0.000) in the second and third trimesters respectively.

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