Estimation of Fetal Weight and Age

Mark A. Curran, M.D., F.A.C.O.G.

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Symphysis–fundal height (SFH)

Measurement of the symphysis-fundal height (SFH) is a common screening method used to estimate the gestational age and fetal growth after 24 weeks gestation. The SFH is measured using a tape placed over the mother's abdomen. The mother's bladder should be empty when the measurement is done. The distance from the top of the pubic bone (symphysis pubis) to the top of the pregnant uterus (fundus) is measured in centimeters (cm). The SFH in centimeters should be equal to the gestational age in weeks. A measurement discrepancy of more 3 cm is suggestive of a fetus with growth problems , an abnormal amniotic fluid level , a transverse lie, a twin pregnancy, or uterine fibroids [1,7].

The sensitivity of SFH measurement for detecting abnormal intrauterine growth was less than 35% in one study [2]. Roex A, et.al. found that the sensitivity of SFH measurement for detecting fetal growth abnormalities could be improved by serial plotting of the SFH on customized charts [3]. A Cochrane review concluded "There is insufficient evidence to determine whether SFH measurement is effective in detecting IUGR. We cannot therefore recommended any change of current practice. Further trials are needed." [4].

When there are factors that increase the risk for intrauterine growth restriction (IUGR) or the SFH is unreliable because of maternal obesity, twin pregnancy, polyhydramnios, or the presence of uterine leiomyomas (fibroids) ultrasonography may be a better screening modality for growth problems in the fetus..

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Ultrasound estimate of gestational age and fetal growth

Prenatally the sonographically estimated fetal weight is used together with weight tables to evaluate fetal growth. Correct evaluation depends on the accuracy of the gestational age being used , the precision of the weight measurements , and using a weight curve that represents the population being studied.

Estimate of gestational age

The American College of Obstetricians and Gynecologists (ACOG) and the Society of Obstetricians and Gynaecologists of Canada recommend ultrasound measurement of the crown rump length (CRL) of the embryo or fetus as the most accurate method to establish or confirm gestational age [21,22]. The SOGC recommends the earliest ultrasound with a crown rump length equivalent to at least 7 weeks (or 10 mm) should be used to determine the gestational age. The SOGC also recommends "...either the best CRL or the average of several satisfactory measurements should be used." The ACOG recommends "The measurement used for dating should be the mean of three discrete CRL measurements when possible..." [21]. It is recommended that crown-rump length be used up to 84 mm, and other parameters be used for measurements > 84 mm [21, 22].

If the pregnancy is the result of in vitro fertilization, embryo age at transfer should be used to establish the estimated due date (EDD).

If the CRL > 84 mm the biparietal diameter (BPD) or head circumference (HC) is the best predictor of gestational age [25] . However, using multiple parameters is superior to using a single parameter to establish a gestational age in the second trimester [23]. Many regression equations are available using various combinations of parameters to estimate the gestational age. Some clinicians use the unweighted mean of the 4 most commonly used biometric parameters ( biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL) ) to establish a gestational age [22]

Ultrasound estimate of fetal weight (EFW)

In practice, common equations for estimating fetal weight include the Hadlock formulas:

Regardless of the formula used the accuracy of the sonographic estimate of the EFW is affected by supoptimal imaging and biological variation . In addition the accuracy of the sonographic estimate decreases with increasing birth weight [26,27] , and tends to be overestimated in pregnancies suspected of being large for gestational age (LGA) and underestimated in pregnancies with preterm premature rupture of membranes (PPROM) and suspected fetal growth restriction (FGR) [10].

Estimated Gestational Age and Fetal Weight Calculator

The calculator linked above uses Hadlock equations to estimate gestational age and fetal weight.

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Evaluation of fetal growth

The average singleton fetus weighs about 80 grams (2.8 ounces) by the end of the first trimester and grows increasingly faster after 22 weeks to reach a maximum growth rate of almost 220 grams (7.8 ounces) per week by 35 weeks '. Growth then slows down and is about 185 grams (6.5 ounces) per week by 40 weeks [17]. Dichorionic twins grow at a similar rate to singletons until about 32 weeks .Thereafter dichorionic twins grow at about 188 grams per week until 37 weeks .Monochorionic twins growth rate growth begins to slow at 28 weeks and averages about 170 grams per week thereafter until 37 weeks [45]

Estimated fetal weight percentile

After obtaining a reliable gestational age and best estimate of fetal weight, a growth chart or weight table may be used to assign an EFW growth percentile.

Singleton, Twin, and Preemie Growth Charts (click images to enlarge)

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The Royal College of Obstetricians and Gynaecologists (RCOG) recommends that a fetal abdominal circumference (AC) or estimated fetal weight (EFW) less than 10th centile can be used to diagnose a small for gestational age (SGA) fetus. ACOG advises newborns whose birth weight is less than the 10th percentile for gestational age are considered small for gestational age (SGA) . Whereas a fetus with a weight that is less than the 10th percentile for its gestational age is considered to have fetal growth restriction (FGR), also known as intrauterine growth restriction (IUGR) [5, 7]. Large for gestational age (LGA) is generally used to describe a birth weight equal to or greater than the 90th percentile for a given gestational age [29]. However, there is not universal agreement on these definitions , and some have proposed the 15th and 97th percentiles as more optimal cut-offs to define SGA and LGA respectively [30].

For comparison , the table below shows the 10th and 90th percentiles for EFW from a Hadlock fetal growth curve and a neonatal birth weight curve using U.S. National Center for Health Statistics data from 2011 .

TABLE 1.

As can be seen from the above Table 1, the Hadlock derived fetal weights are higher than the data derived from the neonatal growth curve until about 37 weeks gestational age. Ott and more recently Solomon et. al., have suggested that IUGR is over-represented in premature deliveries and therefore the fetal growth standards may be more accurate than neonatal growth standards in evaluating the fetus at risk for IUGR [34,24]. Lackman F et al., have also advocated that "...intrauterine growth curves derived from ultrasonographically estimated fetal weight of continuing pregnancies are more appropriate than neonatal growth curves to discriminate fetuses and neonates at higher risk for adverse outcome ." [ 31]

The use of customized fetal growth charts has been proposed to improve the precision in evaluating fetal growth [5]. Table 2 shows data from the National Institute of Child Health and Human Development (NICHD) Fetal Growth Studies which demonstrates the differences in fetal growth between different races and ethnicities [51] . Not all agree that the use of custom growth charts improves prediction of perinatal mortality [6,7, 52,56-58]

TABLE 2

See also: World Health Organization Fetal Growth Calculator

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The Small Fetus

Approximately 70% of infants born below the 10th percentile are not at risk for adverse outcomes. Differences in size are often constitutional (e.g., parental size, ethnicity, fetal sex). About 30% are truly growth restricted and are at increased risk for perinatal morbidity and mortality.

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The Large Fetus

Large for gestational age (LGA) generally implies a birth weight equal to or greater than the 90th percentile for a given gestational age [29] .

Macrosomia "implies growth beyond a specific weight, usually usually 4,000 grams or 4,500 grams regardless of the gestational age" [29, 35, 36] , Boulet SL, et al have advocated dividing macrosomia into 3 grades based on the morbidity and mortality associated with birth weight ranges above 4000 grams [33]

• Grade 1: > 4000 g — increased risks of labor and newborn complications.
• Grade 2: > 4500 g — increased risk of neonatal morbidity.
• Grade 3: > 5000 g — increased risk of infant mortality.

Ye J and colleagues have suggested macrosomia be defined as a birth weight greater than 4300 grams (9 pounds 8 ounces) in Blacks and Hispanics or 4500 grams (9 pounds 15 ounces) in Whites regardless of gestational age. In addition a birth weight greater than the 97th percentile for a given gestational age, irrespective of race is reasonable to define macrosomia [32]

TABLE 3.

Risk factors for macrosomia include , but are not limited to, history of macrosomic baby, excessive weight gain during pregnancy, pre pregnancy maternal BMI, multiparity, male fetus, post term pregnancy, and diabetes [40-44]

Hoopmann and colleagues compared 36 different weight formulas for the estimation of fetal weight for the prediction of macrosomia. They concluded that none of the 36 weight formulas reached a detection rate and false positive rate for fetuses >or=4,500 g that could lead to clinical recommendation [37]. Accuracy of the EFW is reported to be best when examinations are performed within 7 days before delivery [38]. One study found prenatal ultrasound diagnosis of prediction of birth weight to be no more accurate than maternal or clinical predictions of infant birth weight [39]. .

ACOG advises "An accurate diagnosis of macrosomia can be made only by weighing the newborn after delivery." [55]

Reviewed 3/31/2019

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