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Estimation of Fetal Weight and Age

Very small and very large babies have higher mortality and morbidity rates than infants of normal size [53]. Because early detection of growth abnormalities may  help to prevent  fetal demise and manage perinatal complications  more appropriately , monitoring of fetal growth is an important part of antepartum care.

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Symphysis fundal height  ( also known as the fundal height)

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]

 Risk Factors for a Small for Gestational Age Neonate [5,20]

 Odds Ratio  > 3 Odds Ratio > 2
Previous stillbirth Maternal SGA
Antiphospholipid syndrome (APLS ) Chronic hypertension
Diabetes and vascular disease PIH Severe
Unexplained antepartum hemorrhage Preeclampsia
Renal impairment Smoking more than 10 cigarettes per day
Low maternal weight gain Low Pregnancy associated plasma protein-A (PAPP-A) < 0.4 MoM
Paternal SGA Threatened miscarriage
Low maternal weight gain Elevated AFP > 2.0 MoM and hCG > 2.5 MoM
Cocaine use Echogenic bowel found in the fetus
Maternal age > 40 years  
Previous SGA baby  
Daily vigorous exercise  

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.



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 the age of the embryo at the date of transfer should be used to establish the estimated due date (EDD) [21].

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

 In practice the most common equations for calculating the estimated fetal weight (EFW) are reported to be the Shepard and Hadlock formulas [5,8,9]:

Shepard: Log 10 (weight) = -1.7492+ 0.166*BPD +0.046*AC - 2.646*(AC*BPD)/1,000
Hadlock 1: Log 10 (weight) = 1.304+0.05281*Ac+0.1938*FL -0.004*AC*FL
Hadlock 2: Log 10 (weight) = 1.335-0.0034*AC*FL+ 0.0316*BPD+0.0457*AC +0.1623*FL
Hadlock 3: Log 10 (weight) =1.326-0.00326 *AC*FL+0.0107*HC +0.0438*AC + 0.158*FL
Hadlock 4: Log10 (weight) =1.3596 -0.00386* AC * FL+0.0064*HC+0.00061*BPD*AC+ 0.0424*AC+0.174*FL

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].

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Estimated Gestational Age and Fetal Weight Calculator

The calculator below uses Hadlock equations to estimate the gestational age [11,12, 46 ] and the Shepard and Hadlock equations to estimate the  fetal weight 12-15]. Select the parameters and corresponding measured values to use, then press the 'Calculate' button

Select each ultrasound
parameter measured.

Enter measurement (s)  in centimeters (cm )

CRL 

cm [6]

BPD 

cm

HC    

cm

   AC    

cm

FL     

cm

 


 

 


Estimated Due Date (EDD)  and Current Gestational Age 

Enter the date the ultrasound measurement (s) were taken , and the calculated gestational age on the date the ultrasound was performed, then press the 'Calculate' button. The calculator below will give the EDD and current gestational age based on the information you enter.

Enter date sonogram was done

Enter gestational age on day of sonogram  weeks days

 

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 the 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 .
TABLE1.

Gestational  Age in
Weeks

EFW  10th Percentile  

EFW 90th  percentile

Hadlock [17] Duryea [19] Hadlock [17] Duryea [19]
25 652 584 918 938
26 758 637 1068 1080
27 876 719 1234 1260
28 1004 822 1416 1462
29 1145 939 1613 1672
30 1294 1068 1824 1883
31 1453 1214 2049 2101
32 1621 1380 2285 2331
33 1794 1573 2530 2579
34 1973 1793 2781 2846
35 2154 2030 3036 3119
36 2335 2270 3291 3380
37 2513 2500 3543 3612
38 2686 2706 3786 3799
39 2851 2877 4019 3941
40 3004 3005 4234 4057

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] .

TABLE 2

Gestational Age (weeks) EFW   10th Percentile  (grams)   AC  10th Percentile  (cm)
White Black Hispanic Asian White Black Hispanic Asian
20 289 286 279 275 14.06 13.74 13.8 13.92
21 349 342 336 331 15.2 14.81 14.9 15.02
22 417 406 400 394 16.33 15.85 15.98 16.1
23 495 478 473 466 17.44 16.88 17.04 17.15
24 583 559 555 546 18.54 17.87 18.08 18.18
25 682 650 646 637 19.61 18.85 19.1 19.19
26 791 751 749 740 20.66 19.81 20.1 20.18
27 912 863 862 853 21.7 20.76 21.1 21.15
28 1045 985 987 978 22.72 21.72 22.09 22.12
29 1188 1118 1123 1114 23.75 22.69 23.1 23.09
30 1343 1262 1270 1260 24.78 23.68 24.12 24.08
31 1509 1416 1428 1414 25.82 24.68 25.15 25.05
32 1686 1579 1595 1574 26.84 25.68 26.18 26.02
33 1869 1749 1769 1740 27.84 26.65 27.18 26.96
34 2058 1923 1947 1911 28.81 27.58 28.14 27.87
35 2247 2096 2125 2085 29.75 28.45 29.04 28.73
36 2432 2264 2298 2262 30.63 29.27 29.87 29.54
37 2609 2427 2463 2437 31.45 30.06 30.64 30.31
38 2777 2587 2621 2604 32.19 30.83 31.36 31.06
39 2934 2751 2774 2752 32.84 31.6 32.03 31.79
40 3080 2922 2923 2873 33.38 32.4 32.67 32.52

 

See Also  World Health Organization Weight Percentiles Calculator (XLS download)


 

The Small Fetus

70% of infants born below the 10th percentile are not at risk for adverse outcomes [18]. The difference in size between babies is most often due to constitutional causes such as the size of the baby's parents, ethnic background , or the sex of the baby . 30% are truly growth restricted and are at risk for increased perinatal morbidity and mortality.

Fetal Growth Restriction (also known as  Intrauterine Growth Restriction)

The Society of Obstetricians and Gynaecologists of Canada (SGOC) and  the Royal College of Obstetricians and Gynaecologists (RCOG) define  fetal growth restriction (FGR)  as an  estimated weight of < 10th percentile on ultrasound  in a fetus  that, because of a pathologic process, has not attained its biologically determined growth potential [36]

The Prospective Observational Trial to Optimize Pediatric Health in Intrauterine Growth Restriction (IUGR) (PORTO Study) found the presence of an abnormal umbilical artery Doppler ( pulsatility index >95th centile, absent end diastolic flow,  or reversed end-diastolic flow) was significantly associated with adverse outcome*, in FGR fetuses irrespective of EFW percentile or abdominal circumference measurement. A sonographic EFW < 3rd percentile was consistently associated with adverse outcome ; all mortalities had EFW< 3rd centile. [8]

*Adverse perinatal outcome was defined as a composite outcome of intraventricular hemorrhage, periventricular leukomalacia, hypoxic ischemic encephalopathy, necrotizing enterocolitis, bronchopulmonary dysplasia, sepsis, and death.

ACOG recommends "If the ultrasonographically estimated fetal weight is below the 10th percentile for gestational age, further evaluation should be considered, such as amniotic fluid assessment and Doppler blood flow studies of the umbilical artery. Because growth-restricted fetuses have a high incidence of structural and genetic abnormalities, an ultrasonographic examination of fetal anatomy also is recommended if not performed already." [7]

Twins
  • Birth weight discordancy in twins refers to the difference in weight between two twins.
  • Among nonabruption births, Ananth CV et. al., found  a birth weight discordancy of >or=15% among same sex and >or = 30% among different sex twins increased the risk of stillbirths, neonatal deaths, and preterm births [50]
  • Selective intrauterine growth restriction (sIUGR) is present when the fetal weight of one twin is below the 10th percentile [17] in one twin of a monochorionic twin pregnancy while the co-twin is of normal size . sIUGR affects 12 - 25 % of monochorionic (one placenta) twin pregnancies and is thought to be caused by unequal sharing of the placenta by the twins [9]

    Gratacós E, et. .al have suggested fetuses with sIUGR may be classified into three types based on one of three main umbilical artery Doppler waveform patterns, as defined by the characteristics of diastolic flow: positive, persistently absent/reversed or intermittently absent/reversed .

    Classification of sIUGR  [9]:

    • Type I (positive end-diastolic flow in the umbilical artery)
    • Type II (AREDF) : persistently absent or reversed end-diastolic flow
    • Type III (iAREDF). intermittent absent or reversed end-diastolic flow in the absence of fetal breathing.

The Large Fetus

Large for gestational age 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:  Birth weight greater than  4000 g . Increased risks of labor and newborn complications.
  • Grade 2:  Birth weight greater than 4500 g . Increased risk of neonatal morbidity
  • Grade 3:  Birth weight greater than 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

Gestational Age (weeks) EFW   90th Percentile  (grams)   EFW   97th Percentile  (grams)
White Black Hispanic Asian White Black Hispanic Asian
20 381 376 379 373 407 401 407 400
21 460 451 456 447 491 481 489 480
22 551 536 544 532 588 572 584 571
23 654 633 643 628 698 676 691 674
24 771 742 755 737 824 792 812 790
25 903 864 882 859 964 923 948 921
26 1050 1000 1023 997 1121 1069 1100 1069
27 1212 1151 1180 1149 1295 1231 1270 1232
28 1391 1317 1353 1318 1487 1409 1457 1413
29 1587 1498 1543 1501 1697 1604 1662 1609
30 1799 1695 1749 1698 1926 1816 1885 1821
31 2029 1908 1971 1908 2174 2045 2126 2047
32 2276 2135 2209 2129 2441 2290 2384 2284
33 2537 2373 2460 2360 2724 2548 2657 2534
34 2809 2619 2719 2600 3021 2816 2940 2795
35 3088 2868 2983 2851 3326 3087 3229 3067
36 3368 3115 3245 3111 3635 3356 3517 3352
37 3645 3359 3502 3376 3942 3624 3802 3644
38 3918 3605 3756 3637 4246 3896 4086 3933
39 4186 3863 4011 3884 4548 4182 4372 4210
40 4450 4142 4273 4105 4850 4494 4670 4462

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 10/25/2016 by Mark A Curran, M.D.
 

References

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