Intrauterine growth pattern in Butajira HDSS, Southern Ethiopia: BUNMAP pregnancy cohort

Pregnancy is the time in which perinatal outcomes and postnatal growth are established [1]. Fetal size and growth trajectories are important indicators of underlying fetal health during pregnancy. Recently, fetal ultrasound has achieved a central role in diagnosis and management of fetal growth deviations. Small for gestational age (SGA) (smaller in size than the normal for their gestational age) is one of the fetal growth deviations [2]. An estimated fetal weight (EFW) below the 10^th centile is the most commonly used sonographic definition of small for gestational age (SGA) [3].

Detection of an abnormal growth trajectory appears to be a better evaluator of a fetus failing to achieve its growth potential. Serial observation of biometric growth patterns is the most commonly used method to determinate fetal weight [4], and to ascertain expected growth and checking that the fetus is not growth restricted [5]. Ultrasound estimation of fetal weight before birth is very widely used in today’s clinical practice, and, it is essential for the identification and management of high-risk pregnancies [6]. Ultrasound imaging could help for estimating gestational age, fetal size, and early diagnosis of fetal anomalies. This will help for the reduction of mortality rates among mothers and their babies [7].

There are very few ultrasound based multicenter global studies which were mainly conducted by WHO and the INTERGROWTH 21^st Project showing the intra uterine growth pattern and fetal growth chart development from few countries of the world [6, 8]. However, such global studies hide local fetal growth patterns and risk summarizing possibly dissimilar characteristics where dietary habits, adequacy of food intake, health care utilization, and cultural practices are different. Besides, in studies of the Caucasian populations, significant differences in fetal growth were found among different ethnic groups [9]. This shows that racial/ethnic-specific standards improve the precision in evaluating fetal growth.

In Ethiopia, one longitudinal study assessed intrauterine growth patterns in a rural setting and drought-affected population using serial ultrasonographic examination [10]. However, the study was only conducted on fetuses between 24 and 36 weeks of gestation and did not show the full course of growth pattern of fetus. Moreover, the growth pattern in Ethiopia and in Africa at large were not studied sufficiently and the country differences with maternal factors need to adjust the growth charts for local clinical use to increase the accuracy of fetal assessment and to avoid unnecessary obstetric interventions at the time of delivery. Therefore, this study aimed to evaluate the intrauterine growth pattern using prospective longitudinal serial ultrasound measurements in Butajira Ethiopia as the previous WHO and INTERGROWTH 21^st studies focused on global context and among only few countries of the world.

In the BUNMAP Project, a total of 1,860 women were screened and 1,024 of them met the inclusion criteria. Out of the 1,024 women who were enrolled in the study, 196 women exited from the study. Eighty-two women (17 twin pregnancies, four fetuses with congenital anomalies, eight abortions, and 53 women with only single ultrasound measurement in the gestational age 14–38 weeks) were excluded from the analysis (Fig. 1).

We analyzed a total of 2,055 ultrasound scans among 746 singleton fetuses. In the present analysis, we included pregnant women with 2 and above ultrasound measurements from 14–38 weeks of gestation. Growth patterns were analyzed, and growth charts were developed based on the estimated fetal weight (EFW) of 746 singleton fetuses. Percentiles of estimated fetal weight and biometric measurements were analyzed for fetuses above 14 weeks of gestation. Percentiles for small for gestational age was analyzed among fetuses above 22 weeks of gestation.

We described the intrauterine growth pattern in rural community of Ethiopia. We found that the intrauterine growth pattern was similar to the previous studies. The study found that 10% of the fetus were small for gestational age (below the 10^th percentile) based on the Z-score of EFW. We also found that 10.1% of the fetus were small for gestational age based on the Z- score of abdominal circumferences.

Our study shows similar distribution pattern with the international reference standards, However, at 37 weeks of gestation, we found the 5^th,50^th and 95^th percentiles of EFW 2,646, 2,958 and 3,168 gm while the WHO and INTERGROWTH-21^st studies found 2,372, 2,966, 3,538 gm and 2,190, 2,806, 3,480 gm respectively. When we compared with WHO and INTERGROWTH studies, the 95^th percentiles of EFW in this study is lower than them. The difference might be due to the sample size differences between our study and the WHO and INTERGROWTH studies. Participant difference might also bring such differences; INTERGROWTH-21^st and WHO studies took well-nourished participants without known health, environmental, and/or socioeconomic constraints. Besides, living at an altitude lower than 1,500 m was also their inclusion criteria while our study area has an average altitude of 2,500 m above sea level with a range of 1,500 m in the low lands to 3,500 m in the mountainous areas [11]. This high altitude might cause hypoxia which is an important factor to affect fetal development during pregnancy due to limited oxygen to meet the needs of fetal growth and development [21].

The Z- score distributions of BPD, HC, AC and FL in our study ranges from ̵4.9 to + 2.8, ̵ 4.7 to + 4.1, ̵5.7 to + 3.7 and ̵6.1 to + 6.9 respectively. When we fit to the INTERGROWTH-21^st application software, the Z-score distribution of biometric measurements in our study were different and lower than the INTERGROWTH 21^st standards which is between ̵3 & + 3. For example, z-score distribution of BPD in our study is between ̵4.9 and + 2.8. This variation might be due to the measurement difference; in our study BPD was measured from outer to inner diameter of the parietal bone, while INTERGROWTH-21^st measures from outer-to-outer diameter [8]. On average, an estimated increase of 0.06 mm in difference between outer-to-outer and outer-to-inner measurements for every one-week increase in gestational age was observed from the previous study [22]. Other possible reasons for lower Z-score distributions of biometrics in our study might be due to study participant characteristics. The INTERGROWTH-21^st study includes those with adequate nutritional status [23]. This resulted in a group of clinically healthy women, with adequate nutritional status, who by definition were at low risk of fetal growth restriction [8].

The rate of SGA in this study is consistent with the previous study done in the drought affected rural parts of Ethiopia and Tanzania [10, 24]. This similarity between our study and the previous study conducted in the drought affected areas of Ethiopia where more than 40% of the study participants were undernourished at the start of the study, might be explained by placental adaptation; maternal undernutrition is associated with degenerative placenta changes. However, degenerative placental changes may not impair the placenta’s transport capacity sufficiently to alter fetal nutrition and were not thereby associated with reduction in fetal growth [25].

In our study, both EFW and AC z-scores below the 10^th percentile gave similar rate of SGA. This indicates, AC and EFW below the 10^th percentile can be used to diagnose small for gestational age. Different research also examined the ability of AC in detecting SGA and fetal growth restriction. They found that AC below the 10^th percentile can be used to diagnose FGR [26, 27].

We calculated the fetal biometrics ratios to see the fetal body proportion. A literature says that if the 90^th percentile of the FL/AC ratio ˃23.5%, the fetus is growth retarded [28]. In our study the 90^th percentile is above the recommended values which is 24.4% and suggestive of fetal growth retardation. Hohler and Quetel also stated that a FL/BPD ratio of 79 ± 8% after 22 weeks of gestation is normal. Values above the specified number considered as microcephaly and lesser values considered as short limbed dwarfism [29]. In our case we found that 75.3 ± 5.5% FL/BPD ratio which can be suggestive of short limbed dwarfism. When we compared the FL/HC ratio with the WHO fetal growth charts, our finding is much lower than the WHO standards.

Our study evaluated the intrauterine growth patten using ultrasound measurements. We found that the fetal growth pattern in our study is similar with the previous study conducted in drought affected areas of Ethiopia, the WHO and INTERGROWTH studies. Our study also found that 10% of the fetus were small for their gestational age. Therefore, we recommend subsequent follow up studies to identify the causes of growth restriction and small fetal size in the study area to confirm whether the small fetal size is due to other causes or the nature of the population in the study area.