There are several techniques for clinical estimation of foetal weight.3,7,42 Many of the clinical techniques of foetal weight estimation have also been reviewed.7 The clinical method include palpation and use of formulae to estimate the foetal weight.1,3,7,39 Various calculations and formulae based on measuring uterine fundal height above symphysis pubis have been developed.
2.9.1 TACTILE ASSESSMENT OF FOETAL SIZE
Tactile assessment of foetal size is the oldest technique for assessing foetal weight and is also referred to as clinical palpation or Leopold maneuver. It involves manual assessment of foetal size by the Obstetrician. Worldwide, this method is used extensively because it is both convenient and virtually costless. However, it is a subjective method associated with notable predictive errors.1,3,7
2.9.2 ADVANCES MADE IN CLINICAL ESTIMATION OF FOETAL WEIGHT
The first method for estimating foetal weight was that of Poulos and Langstadt.57 They achieved accuracy within around 250g in 69% of estimations, with a correlation coefficient (r) of 0.62 to 0.70. This was the first report of external measurements to predict birth weight using external and trans-rectal measurement of the different axes of the uterus. Poulos and Langstadt based their estimations on two theories. The first was the fundamental physical law
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stating that for a given homogeneous mass, the weight of the mass (W) is directly proportional to its volume (V), where the density (D) is constant (W = DV). In applying this to the birth weight of an infant and the volume of a uterus, it is obvious that the volume of the uterus is not accurately known, and certain assumptions must be made. This brought about the second theory on the theoretical volume of the uterus. The volume of the uterus was assumed to be either a sphere, or an ellipsoid (an ellipse rotated around its long axis). Based on these considerations, Poulos and Langstadt suggested a foetal weight formula of: Birth weight (g) =1870 + 0.11D 3 ± 250, with D being the mean of the transverse and longitudinal uterine diameters in cm.57
This provided the estimates described above. All subsequent measurement formulas depend to a greater or lesser extent on Poulos and Langstadt's mathematical assumptions.42,57 The second method was the use of symphysis fundal height (SFH) measurement as first described by Johnson and Toshach,42 who reported accuracy within 240g in 50.5% of 200 women examined. They found that foetal weight estimation was affected also by head descent and maternal obesity, and suggested the birth weight formula, using the imperial system. Their formula was: Birth weight = 7 pounds, 8 ounces + [(M + S – O – 34) × (5.52 ounces)] where: M = height of fundus in cm S = station, subtracting 1cm for minus stations, adding 1 cm for plus stations. O = obesity, subtracting 1cm for women weighing over 200 pounds (91kg).
In a subsequent study, Johnson presented a simplified formula that took into account the more modern metric system. For a foetus with a non-engaged head, it was suggested that:
Birth weight in g = (SFH measurement in cm – 13) × (155). Despite claims of reasonable accuracy, these authors did not validate their findings in repeat studies.
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The Johnson's formula.1,3,7,42 for estimation of foetal weight in vertex presentation is as follows; Foetal weight (g) = FH (cm) n × 155. Where FH = fundal height and n = 12 if vertex is above ischial spine or 11 if vertex is below ischial spine. If a patient weighs more than 91Kg, 1cm is subtracted from the fundal height.
Thirdly, estimated foetal weight in grammes can also be determined by multiplying the longitudinal diameter of the uterus by the square of its transverse diameter and a factor of 1.44, then dividing the product by 2.
Fourth was Dawn's method with formula as: Estimated foetal weight = longitudinal diameter of the uterus × (transverse diameter) 2 × 1.44/2. Measurement was made with pelvimeter and if double abdominal thickness was more than 3cm, the excess was subtracted from the transverse diameter and half of the excess from longitudinal diameter.42
Ojwang et al. used the product of symphysio-fundal height (SFH) and abdominal girth (AG) measurement at various levels in centimetres above the symphysis pubis in obtaining a fairly acceptable predictive value but with considerable variation from the mean.42
Ojwang's method, was further simplified by Dare et al. at Ile-Ife, Nigeria, in 1990, who used the product of SFH and abdominal girth at the level of the umbilicus measured in centimeters, that is, (SFH × AG) grammes. The result was expressed in grammes to estimate foetal weight at term in-utero, and the estimate correlated well with birth weight.31,42
The formula; birth weight (g) = gestational age (d) × [9.36 + 0.262 × foetal sex + 0.000237 × maternal height (cm) × maternal weight at 26 weeks (kg) + (4.81 × maternal weight gain rate (kg/d) × (parity + 1)], where foetal sex is equal to + 1 for male, -1 for female and 0 for
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unknown sex, and gestational age is equal to days since onset of last normal menses which equals the conception age (d)+14.42
These methods have been recommended as a good way of foetal weight estimation in low resource communities where ultrasonography or the skills for it may not be available.42
Bothner et al showed good correlation between intra-partum SFH measurement and birth weight (r = 0.56).42 Their equation was: Birth weight = (SFH – fifths – 20) × (300). The study was done in established labour at gestational age ranges of 27–44 weeks by dates, and 25–42 weeks by ultrasound. The finding suggested that the level of the foetal head and status of membranes either intact or ruptured might affect SFH measurement. However, they concluded that SFH measurement for foetal weight estimation was not clinically useful using their formula.42 Although Jeffery et al did a similar work and reported better correlation (r = 0.74).42 They could not suggest a simpler formula.
The maternal-characteristics equation stated as; Birth weight (in grams) = −3044 + gestational (in days) × {[23.6+ (0.243 × foetal sex)+ (0.000281 × material height in centimeters × maternal weight at 26 weeks in kilograms)+ (3.11 × maternal weight-gain rate in kilograms per day × [parity + 1]) − (0.318 × maternal hemoglobin concentration in grams per deciliter)}, where foetal sex = +1 for male, −1 for female, or 0 for unknown and gestational age = days since the onset of the last normal menses, which equals the conception age (in days) +14.42
Prospective use of this maternal-characteristics equation yields a correlation of 0.59 between predicted and actual birth weights, with a mean absolute prediction error of 275 ± 229 g (8.0% ±7.1 of actual birth weight). In 86% of cases, the birth weight predictions were
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accurate to within ± 15%, and in 70% of cases, they were accurate to within ± 10% of actual birth weight.42
The Kongnyuy – Mbu's method uses only FH to estimate foetal weight (FW) in grams [Estimated FW = 3(FH)2]. The two authors working in the biggest maternity in Cameroon discovered that the AC of a pregnant Cameroonian woman at term is thrice the fundal height and they derived the formula from the FH × AC used routinely to estimate foetal weights in the service. The authors explained that AC is influenced by the body mass index (BMI) while fundal height is not. The inexpensive and easy availability of the non-elastic tape makes it attractive for use in foetal weight estimation in developing countries.42 Most recent studies done in Nigeria are still based on Dare's formula and there is a need to explore the reliability of other formula and compare them with Dare's formulae. Some studies have compared sonographic estimates and Johnson's formula and agreed that it has a high level of accuracy especially in foetuses above 3Kg.1,3,7,37,38, There is need to validate these claims in recent researches. Dare et al validated the formula in Nigeria.31
Other methods are based on SFH alone with different correlations.36,58 However, a lot of researchers have felt that SFH measurement alone is not sensitive or specific enough for practical use.42 Maternal obesity, level of foetal head and membrane status or a full bladder may all increase the SFH.
Clear descriptions of symphysio-fundal height measurement have been suggested by Westin, Theron and Bothner.42 Measurement should be in a supine position with an empty bladder, between uterine contractions. Two measurements should ideally be made and the average of these taken as the SFH.
The correlation between clinical EFW and actual BW is overall weak, particularly in patients with macrosomic fetuses: However, BMI, admission diagnosis and foetal station do not have
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a significant impact.3 A prudent advice is perhaps that of workers who also found a significant error estimating foetal weight by ultrasound. Therefore, they concluded that depending only on the foetal ultrasound for the estimation of foetal weight can lead to unnecessary obstetrical intervention. It is thus necessary to correlate the ultrasound findings with clinical examination.36
2.9.3 CLINICAL RISK FACTOR ASSESSMENT
Quantitative assessment of clinical risk factors can be valuable in predicting deviations in foetal weight.1,3,4,7 Foetal macrosomia is associated with some clinical risk factors.
2.9.4 MATERNAL SELF-ESTIMATION
Another method for estimating foetal weight is maternal self-estimation. Self-estimates of foetal weight in multiparous women were comparably accurate to findings on clinical palpation in predicting foetal weight and abnormally large foetuses at term.1,3,4,7,19
Several studies have been done to compare clinical estimated foetal weight and actual birth weight.1,3,7 Ashrafganjooei et al in a prospective study of 246 women in Iran found the mean actual birth weight to be 3339 ± 443g, while the mean clinical estimated weight was 3321±
449g.2 Abdulrazak et al conducted a cross-sectional study at Al-Elwiya- Bahgdad compared the clinical estimated foetal weight with actual birth weight. The mean actual birth weight was found to be 3376±487g, and the mean clinical estimated foetal weight was 3457 ± 560Kg, (P=value 0.276, CI=65.3-22.7) The clinical foetal weight estimation was a relatively accurate method of foetal weight estimation when actual birth weight is more than 3Kg.20 (P value 0.907) Njoku et al in a prospective cross-sectional study in Calabar-Nigeria found the mean birth weight to be 3242+/- 508g. The mean clinical estimated weight was 3541+/- 633g, with P- Value of 0.695.1
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2.10 SENSITIVITY AND SPECIFICITY OF CLINICAL ESTIMATION OF FOETAL