Background
Maternal and perinatal mortalities in sub-Saharan countries are among the highest in the world. In Cameroon the maternal mortality rate is 430 per 100 000 live births and the infant mortality rate 87 per 1000 under one year)
Last menstrual period (LMP) is simple and the most common method of calculating GA. However, 45–68% of pregnant women have been reported to have irregular periods or uncertain information regarding their LMP. The ultrasonographic measurements of the fetal BPD is a more reliable method, predicting date of spontaneous delivery with greater certainty than even a certain LMP
Biparietal diameter (BPD) is the most commonly used ultrasound measurement for fetal age assessment. Fetal age assessment in the second trimester can also be based on head circumference (HC) and femur length (FL). These methods are less influenced by maternal and fetal factors such as parity, age and fetal gender
The African population is composed of a variety of ethnic groups, differing considerably from each other. Some studies suggest that population differences in fetal biometry are negligible and that separate studies are not essential
The aim of the present study was therefore to establish reference values for fetal age assessment in Cameroon using two different ethnic groups. We also wanted to determine the effect of maternal morphometry on the age assessment and to compare these new reference charts with other relevant charts.
Methods
This was a prospective cross sectional study of 200 pregnant women belonging to two different ethnic groups in the northern part of Cameroon: the Fulani people who are slender, and the Kirdi people who are in general stocky (Table
Comparison between Fulani (N = 96) and Kirdi (N = 104) according to height, weight, body mass index (BMI), maternal age (A = ANOVA test).
Height
Mean
95% Confidence Interval
Fulani
1.57
1.56
1.58
Kirdi
1.595
1.585
1.605
Total
1.58
1.57
1.59
Weight
Mean
95% Confidence Interval
Fulani
59.93
57.90
61.95
Kirdi
65.87
63.93
67.82
Total
62.90
61.50
64.30
BMI
Mean
95% Confidence Interval
Fulani
24.03
23.26
24.80
Kirdi
25.83
25.09
26.57
Total
24.93
24.40
25.46
Age
Mean
95% Confidence Interval
Fulani
25.32
24.19
26.45
Kirdi
27.59
26.50
28.67
Total
26.46
26.67
27.24
One investigator (DS) trained at a Norwegian university hospital did all the ultrasound examinations using Shimasonic (Shimadzu) SDL-300, Japan, with a 3.5 MHz curvilinear probe.
Fetal head measurements were obtained in a horizintal section at the level of the thalamus and the cavum septi pellucidi
Gestational age was calculated from the first day of the last menstrual period, and corrected for cycle length; i.e. corresponding number of days were added or subtracted according to menstrual cycle length shorter or longer than 28 days, respectively.
Statistics
The sample size was determined based on the power calculation and design of a corresponding previous study
Results
Table
Comparison between Fulani (N = 96) and Kirdi (N = 104) according to parity (B= Chi-square test)
Parity
Fulani
Kirdi
Total
n
%
N
%
n
%
0
22
22.92
24
23.08
46
24.21
Chi-square test:
1
24
25.00
26
25.00
50
26.32
2
14
14.58
15
14.42
29
15.26
3
13
13.54
10
9.62
23
12.11
4
9
9.38
9
8.65
18
9.47
5
14
14.58
20
19.23
34
17.90
BPD, OFD and HC were successfully determined in all participants, while in 18 cases visualisation of the FL was not possible during early pregnancy. For the ethnic groups combined, raw data with fitted 2.5th, 50th and 97.5th centiles and 95% CI for mean gestational age as functions of BPDoo, BPDoi, HC and FL are presented in Figures
Gestational age assessment by biparietal diameter outer-outer (BPDoo)
Centiles
50th
2.5th
97.5th
BPDoo (mm)
weeks
days
weeks
days
weeks
days
20
11
6
10
6
12
6
21
12
1
11
1
13
0
22
12
3
11
3
13
2
23
12
5
11
5
13
4
24
13
0
12
0
13
6
25
13
2
12
2
14
1
26
13
4
12
4
14
3
27
13
5
12
6
14
5
28
14
0
13
1
15
0
29
14
2
13
3
15
2
30
14
4
13
5
15
4
31
14
6
14
0
15
6
32
15
1
14
2
16
1
33
15
3
14
4
16
3
34
15
5
14
6
16
5
35
16
0
15
1
16
6
36
16
2
15
3
17
1
37
16
4
15
5
17
3
38
16
6
16
0
17
5
39
17
1
16
2
18
0
40
17
3
16
4
18
2
41
17
5
16
6
18
4
42
18
0
17
1
18
6
43
18
2
17
3
19
1
44
18
4
17
5
19
3
45
18
6
18
0
19
5
46
19
1
18
2
20
0
47
19
3
18
4
20
2
48
19
5
18
6
20
4
49
20
0
19
1
20
6
50
20
2
19
3
21
1
51
20
4
19
5
21
3
52
20
6
20
0
21
5
53
21
1
20
2
22
0
54
21
3
20
4
22
2
55
21
5
20
6
22
4
56
22
0
21
1
22
6
57
22
2
21
3
23
1
58
22
4
21
5
23
3
Gestational age asssessment by biparietal diameter outer-inner (BPDoi)
Centiles
50th
2.5th
97.5th
BPDoi (mm)
weeks
days
weeks
days
weeks
days
20
12
2
11
3
13
1
21
12
4
11
5
13
3
22
12
6
12
0
13
5
23
13
1
12
2
14
0
24
13
3
12
4
14
2
25
13
5
12
6
14
4
26
14
0
13
1
14
6
27
14
2
13
3
15
1
28
14
4
13
5
15
3
29
14
6
14
0
15
5
30
15
1
14
2
16
0
31
15
3
14
4
16
2
32
15
5
14
6
16
4
33
16
0
15
1
16
6
34
16
2
15
3
17
1
35
16
4
15
5
17
3
36
16
6
16
0
17
5
37
17
1
16
2
17
6
38
17
3
16
4
18
1
39
17
5
16
6
18
3
40
18
0
17
1
18
5
41
18
2
17
3
19
0
42
18
4
17
5
19
2
43
18
6
18
0
19
4
44
19
1
18
2
19
6
45
19
3
18
4
20
1
46
19
5
18
6
20
3
47
20
0
19
1
20
5
48
20
2
19
3
21
0
49
20
4
19
5
21
2
50
20
6
20
0
21
4
51
21
1
20
2
21
6
52
21
3
20
4
22
1
53
21
5
20
6
22
3
54
22
0
21
1
22
5
55
22
2
21
3
23
1
56
22
4
21
5
23
3
57
22
6
22
0
23
5
58
23
1
22
2
24
0
Gestational age ssessment by head circumference (HC)
Centiles
Centiles
50th
2.5th
97.5th
50th
2.5th
97.5th
HC (mm)
weeks
days
weeks
days
weeks
days
HC (mm)
weeks
days
weeks
days
weeks
days
70
12
2
11
2
13
3
140
18
0
17
0
19
0
72
12
3
11
3
13
4
142
18
1
17
1
19
1
74
12
4
11
4
13
5
144
18
2
17
2
19
2
76
12
5
11
5
13
6
146
18
3
17
3
19
3
78
12
6
11
6
14
0
148
18
4
17
4
19
4
80
13
0
12
0
14
1
150
18
5
17
5
19
5
82
13
1
12
1
14
2
152
19
0
17
6
20
0
84
13
3
12
2
14
3
154
19
1
18
1
20
1
86
13
4
12
3
14
4
156
19
2
18
2
20
2
88
13
5
12
4
14
5
158
19
3
18
3
20
3
90
13
6
12
5
14
6
160
19
4
18
4
20
4
92
14
0
12
6
15
0
162
19
5
18
5
20
5
94
14
1
13
1
15
1
164
19
6
18
6
20
6
96
14
2
13
2
15
3
166
20
0
19
0
21
0
98
14
3
13
3
15
4
168
20
1
19
1
21
1
100
14
5
13
4
15
5
170
20
2
19
2
21
2
102
14
6
13
5
15
6
172
20
3
19
3
21
3
104
15
0
13
6
16
0
174
20
4
19
4
21
4
106
15
1
14
1
16
1
176
20
5
19
5
21
5
108
15
2
14
2
16
2
178
20
6
19
6
21
6
110
15
3
14
3
16
4
180
21
0
20
0
22
0
112
15
4
14
4
16
5
182
21
1
20
1
22
1
114
15
6
14
5
16
6
184
21
2
20
2
22
2
116
16
0
15
0
17
0
186
21
3
20
3
22
3
118
16
1
15
1
17
1
188
21
4
20
4
22
4
120
16
2
15
2
17
2
190
21
5
20
5
22
4
122
16
3
15
3
17
3
192
21
5
20
6
22
5
124
16
5
15
4
17
5
194
21
6
20
6
22
6
126
16
6
15
5
17
6
196
22
0
21
0
23
0
128
17
0
16
0
18
0
198
22
1
21
1
23
1
130
17
1
16
1
18
1
200
22
2
21
2
23
2
132
17
2
16
2
18
2
202
22
3
21
3
23
2
134
17
3
16
3
18
3
204
22
3
21
3
23
3
136
17
4
16
4
18
5
206
22
4
21
4
23
4
138
17
6
16
6
18
6
208
22
5
21
5
23
5
210
22
5
21
6
23
5
Gestational age assessment by femur length (FL)
Centiles
50th
2.5th
97.5th
FL (mm)
weeks
days
Weeks
days
weeks
days
11
11
5
12
0
13
6
12
12
1
12
4
14
3
13
12
4
13
0
14
6
14
13
0
13
3
15
2
15
13
3
13
5
15
5
16
13
5
14
1
16
0
17
14
1
14
4
16
3
18
14
4
14
6
16
6
19
14
6
15
2
17
1
20
15
2
15
4
17
4
21
15
4
16
0
17
6
22
15
6
16
2
18
2
23
16
2
16
5
18
4
24
16
4
17
0
19
0
25
16
6
17
2
19
2
26
17
1
17
4
19
4
27
17
4
18
0
20
0
28
17
6
18
2
20
2
29
18
1
18
4
20
4
30
18
3
18
6
20
6
31
18
5
19
1
21
2
32
19
0
19
3
21
4
33
19
2
19
5
21
6
34
19
4
20
1
22
1
35
19
6
20
3
22
3
36
20
2
20
5
22
5
37
20
4
21
0
23
1
38
20
6
21
2
23
3
39
21
1
21
4
23
5
40
21
3
21
6
24
0
Raw data with fitted centiles of gestational age (2.5th, 50th and 97.5th) by biparietal diameter (outer-outer) and 95% CI for the mean
Raw data with fitted centiles of gestational age (2.5th, 50th and 97.5th) by biparietal diameter (outer-outer) and 95% CI for the mean. Red dots are Fulani and blue dots Kirdi.
Raw data with fitted centiles of gestational age (2.5th, 50th and 97.5th) by biparietal diameter (outer-inner)) and 95% CI for the mean
Raw data with fitted centiles of gestational age (2.5th, 50th and 97.5th) by biparietal diameter (outer-inner)) and 95% CI for the mean.
Raw data with fitted centiles of gestational age (2.5th, 50th and 97.5th) by head circumference and 95% CI for the mean
Raw data with fitted centiles of gestational age (2.5th, 50th and 97.5th) by head circumference and 95% CI for the mean.
Raw data with fitted centiles of gestational age (2.5th, 50th and 97.5%) by femur length and 95% CI for the mean
Raw data with fitted centiles of gestational age (2.5th, 50th and 97.5%) by femur length and 95% CI for the mean.
In supplementary analyzes, we compared z-scores of gestational age in the two ethnical groups. For both groups, mean z-scores for each anatomical parameter was non-different from zero (General Linear Models, SPSS). Nor did inclusion of ethnicity as an independent variable in the functions, which describe the estimated mean gestational age, reveal significant effects. Mean z-scores of gestational age as function of HC in Fulani and Kirdi women were -0.02 (95% CI: -0.22, 0.18), SD 0.98, and 0.02 (95% CI: -0.19, 0.22), SD 1.05, respectively. Correspondingly, mean z-scores of gestational age according to BPDoo in Fulani and Kirdi were 0.00 (95% CI: -0.19, 0.19), SD 0.92, and 0.00 (95% CI: -0.22, 0.23), SD 1.15, respectively. For BPDoo mean gestational age z-scores in Fulani and Kirdi women were -0.02 (95% CI: -0.21, 0.17), SD 0.94 and 0.02 (95% CI: -0.21, 0.25), SD 1.18, respectively. For FL mean gestational age z-scores in Fulani and Kirdi women were -0.12 (95% CI: -0.39, 0.15), SD 1.30 and 0.12 (95% CI: -0.08, 0.32), SD 0.97, respectively.
Prediction of gestational age from BPD (22–59 mm) was fairly similar in three studies using outer-outer measurement technique (Fig
Uncertainty of gestational age assessment, expressed as the distance between the 5th and 95th centile, when using outer-outer biparietal diameter (BPDoo), head circumference (HC) and femur length (FL) based on the present study, the study of Altman and Chitty
Present study
Altman and Chitty
Johnsen
Measurement
50th centile
Uncertainty
50th centile
Uncertainty
50th centile
Uncertainty
(mm)
(weeks + days)
(± days)
(weeks + days)
(± days)
(weeks + days)
(± days)
BPDoo
22
12 + 3
6
12 + 4
7
12 + 3
6
50
20 + 2
5
20 + 3
13
20 + 2
9
HC
90
13 + 6
6
13 + 2
5
13 + 3
6
180
21 + 0
6
20 + 5
10
20 + 4
8
FL
14
14 + 1
7
14 + 1
7
14 + 3
6
32
20 + 2
8
20 + 0
11
20 + 2
8
The 50th centile for biparietal diameter (outer-outer) with 95% confidence interval in the present study compared with those of Altman and Chitty (blue line)
The 50th centile for biparietal diameter (outer-outer) with 95% confidence interval in the present study compared with those of Altman and Chitty (blue line)
The 50th centile for head circumference with 95% confidence interval in the present study compared with those of Altman and Chitty (blue line)
The 50th centile for head circumference with 95% confidence interval in the present study compared with those of Altman and Chitty (blue line)
The 50th centile for femur length with 95% confidence interval in the present study compared with those of Altman and Chitty (blue line)
The 50th centile for femur length with 95% confidence interval in the present study compared with those of Altman and Chitty (blue line)
In Tables
Effects of maternal ethnicity, weight, height, body mass index (BMI), parity, and age on biparietal diameter outer – outer (BPDoo)
Maternal factor
Mean
95%CI
SD
p-value
Ethnicity
0.82
Fulani
40.52
40.19
40.85
9.8
Kirdi
40.47
40.15
40.78
11.2
Weight (centiles)
0.39
< 10
40.30
39.64
40.97
10.1
10–90
40.44
40.18
40.69
10.6
90+
41.11
40.40
41.83
10.6
Height (centiles)
0.83
< 10
40.47
39.77
41.17
9.7
10–90
40.54
40.27
40.80
10.7
90+
40.25
39.64
40.86
10.6
BMI (centiles)
0.33
< 10
40.47
39.74
41.19
8.5
10–90
40.44
40.19
40.69
10.7
90+
40.91
40.19
41.63
10.7
Parity
0.01
0
38.67
37.97
39.38
11.4
1
39.04
38.39
39.69
10.1
2
39.70
38.85
40.55
11.2
3
40.41
39.55
41.27
11.5
4
39.45
38.51
40.40
10.3
5+
39.99
39.12
40.87
9.3
Age
0.10
-19
40.25
39.16
41.35
12.3
20–24
39.98
39.37
40.58
10.4
25–29
39.53
39.01
40.06
10.0
30–34
38.55
37.81
39.28
11.9
35+
39.41
38.28
40.55
9.7
Effects of maternal ethnicity, weight, height, body mass index (BMI), parity, and age on head circumference (HC)
Maternal factor
Mean
95%CI
SD
Ethnicity
0.89
Fulani
136.59
135.21
137.97
36.6
Kirdi
136.46
135.13
137.78
40.4
Weight (centiles)
0.22
< 10
135.47
132.66
138.27
39.4
10–90
136.30
135.23
137.37
38.2
90+
139.45
136.43
142.46
39.0
Height (centiles)
0.58
< 10
136.10
133.17
139.03
37.5
10–90
137.05
135.95
138.14
39.6
90+
134.00
131.46
136.55
39.6
BMI (centiles)
0.20
< 10
135.68
132.62
138.73
32.9
10–90
136.42
135.35
137.49
38.8
90+
138.19
135.16
141.22
38.1
Parity
0.27
0
132.01
129.21
134.81
41.8
1
131.25
128.68
133.82
36.3
2
132.64
129.26
136.02
40.9
3
135.78
132.38
139.17
40.8
4
132.05
128.30
135.81
40.0
5+
134.50
131.04
137.97
34.0
Age
0.34
-19
134.81
130.45
139.16
45.6
20–24
134.26
131.85
136.66
38.1
25–29
132.09
130.00
134.17
36.7
30–34
132.44
129.52
135.36
43.1
35+
131.60
127.09
136.11
33.9
Analysis of variance (ANOVA), adjustments for gestational age.
Parity and age were adjusted for each other.
Effects of maternal ethnicity, weight, height, body mass index (BMI), parity, and age on fetal femur length (FL)
Maternal factor
Mean
95%CI
SD
Ethnicity
0.24
Fulani
24.53
22.89
26.17
7.7
Kirdi
25.79
24.18
27.41
8.0
Weight (centiles)
0.20
< 10
23.33
19.95
26.72
8.4
10–90
25.26
23.96
26.57
7.8
90+
26.53
22.97
30.08
7.8
Height (centiles)
0.22
< 10
24.05
20.66
27.44
8.4
10–90
25.18
23.84
26.52
7.8
90+
26.04
22.99
29.09
7.6
BMI (centiles)
0.60
< 10
21.61
17.99
25.24
7.3
10–90
25.45
24.17
26.73
7.7
90+
26.42
22.89
29.95
8.3
Parity
0.11
0
23.95
23.02
24.88
8.7
1
24.21
23.37
25.05
7.7
2
24.77
23.62
25.92
7.7
3
23.88
22.77
24.99
5.6
4
24.63
23.40
25.86
8.2
5+
25.63
24.53
26.73
7.4
Age
0.17
-19
25.32
23.91
26.74
8.6
20–24
24.91
24.15
25.66
7.8
25–29
24.62
23.98
25.27
7.9
30–34
23.99
23.03
24.96
8.3
35+
23.71
22.27
25.15
7.1
Analysis of variance (ANOVA), adjustments for gestational age.
Parity and age were adjusted for each other.
The intra-observer variation, calculated as the coefficient of variation for BPDoo, BPDoi, HC and FL was 2.7 (95% CI 2.5–2.9), 2.7 (2.6–2.9), 3.8 (2.8–3.3) and 3.1 (2.8–3.3), respectively. The corresponding intra-class correlation was 99.1% (95% CI 98.9–99.3), 99.2 (99.0–99.4), 98.4 (97.9–98.7) and 99.3 (99.11–99.4), respectively.
Discussion
We have established reference charts for gestational age assessment using three fetal ultrasonographic measurements in an African population composed of two different ethnic groups. We have shown that significant morphometric ethnic differences had no significant influence on gestational age assessment. The charts are in agreement with European charts based on corresponding design and statistics.
Although ethnic groups (Kirdi and Fulani) differed significantly with respect to maternal age, height, weight and BMI (Table
As for FL there was no difference compared with the Norwegian study (95% CI overlapped). These two studies used identical insonation and measurement techniques as all ultrasound operators had been trained in the same unit, but then applied this technique using different machines in different countries. In general, FL charts vary more from study to study than the fetal head biometry, which probably reflects an uncertainty in defining the landmarks for the femoral diaphysis. We believe the small differences compared with the British charts are due to measurement technique, since study design and statistical methods were otherwise identical.
We have previously shown that morphometric differences at 18 weeks of gestation are related to body composition at birth
The impact of weight, height, maternal age, and BMI on fetal biometry, was non-existent in the present study or so small that it can be ignored before 22 weeks of gestation. There was a significant effect of parity on BPDoo, but since there was no effect on HC and FL it may well be due to chance. A study of normal pregnancies in Papua New Guinea found that fetal biometry was hardly affected by socio-demographic characteristics, weight gain during pregnancy or the height of the mother
In the present study all participants were married and lived with their husband, a sign of couple's stability. Although most of the women were housewives and dependent on their husbands, at least one member of the couple had income. This does not reflect the general population where more than 30% are believed to be unemployed, and this suggests that our study population was skewed. The fact that we included only women who knew their LMP probably augmented this skewed distribution as destitute pregnant women tend not to know their LMP and are therefore more likely to be excluded from the study. Recently established WHO standards for infant growth included children from optimal socioeconomic backgrounds at different locations around the world
This study can be criticised for lacking perinatal data. We could not collect the outcomes of pregnancy as this would have necessitated the investigator's presence at the study site for a longer period. Home birth is common and would have resulted in numerous dropouts. Secondly, if growth deviation or other complications had occurred, we would not have excluded these participants
Conclusion
In countries such as Cameroon where the illiteracy is very high (68%)
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
DS contributed to conception and design of the study, acquisition of data, interpretation of data, revision of the manuscript, and approved the final version. TK contributed to conception and design of the study, analysis and interpretation of data, helped to draft the manuscript, and approved the final version. SR contributed to conception and design of the study, analysis and interpretation of data, helped to draft the manuscript, and approved the final version. SLJ contributed to analysis and interpretation of data, drafted the manuscript, and approved the final version.