Estimation of Fetal Weight

Why is fetal weight important?

Estimation of Fetal Weight and Age
Mark A Curran, M.D., F.A.C.O.G.

All calculations must be confirmed before use. The suggested results are not a substitute for clinical
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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  {amp}gt; 3 Odds Ratio {amp}gt; 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) {amp}lt; 0.4
MoM
Paternal SGA Threatened miscarriage
Low maternal weight gain Elevated AFP {amp}gt; 2.0 MoM and hCG {amp}gt; 2.5 MoM
Cocaine use
Echogenic bowel found in the fetus
Maternal age {amp}gt; 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 {amp}gt;
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
theestimated due date(EDD)
[21].

If the CRL {amp}gt; 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

From early in pregnancy, babies grow at different rates, so these numbers are merely averages. Your baby’s actual length and weight may vary substantially.

Don’t worry too much if an ultrasound indicates that your baby is much smaller or larger. (Your practitioner will let you know if it’s time to worry about how big your baby is.) By full-term, your baby may end up weighing less than 5 pounds or more than 9.

Until about 20 weeks, babies are measured from the crown (or top) of the head to the rump (or bottom). This is because a baby’s legs are curled up against his torso during the first half of pregnancy and very hard to measure.

After that, babies are measured from head to toe.

See our pregnancy timing article to learn how the weeks of pregnancy are counted and your baby’s gestational age is determined. If you don’t know your due date, use our due date calculator to find out.

Gestational age Length (US) Weight (US) Length (cm) Mass (g)
(crown to rump) (crown to rump)
8 weeks 0.63 inch 0.04 ounce 1.6 cm 1 gram
9 weeks 0.90 inch 0.07 ounce 2.3 cm 2 grams
10 weeks 1.22 inch 0.14 ounce 3.1 cm 4 grams
11 weeks 1.61 inch 0.25 ounce 4.1 cm 7 grams
12 weeks 2.13 inches 0.49 ounce 5.4 cm 14 grams
13 weeks 2.91 inches 0.81 ounce 7.4 cm 23 grams
14 weeks 3.42 inches 1.52 ounce 8.7 cm 43 grams
15 weeks 3.98 inches 2.47 ounces 10.1 cm 70 grams
16 weeks 4.57 inches 3.53 ounces 11.6 cm 100 grams
17 weeks 5.12 inches 4.94 ounces 13 cm 140 grams
18 weeks 5.59 inches 6.70 ounces 14.2 cm 190 grams
19 weeks 6.02 inches 8.47 ounces 15.3 cm 240 grams
20 weeks 6.46 inches 10.58 ounces 16.4 cm 300 grams
(crown to heel) (crown to heel)
20 weeks 10.08 inches 10.58 ounces 25.6 cm 300 grams
21 weeks 10.51 inches 12.70 ounces 26.7 cm 360 grams
22 weeks 10.94 inches 15.17 ounces 27.8 cm 430 grams
23 weeks 11.38 inches 1.10 pound 28.9 cm 501 grams
24 weeks 11.81 inches 1.32 pound 30 cm 600 grams
25 weeks 13.62 inches 1.46 pound 34.6 cm 660 grams
26 weeks 14.02 inches 1.68 pound 35.6 cm 760 grams
27 weeks 14.41 inches 1.93 pound 36.6 cm 875 grams
28 weeks 14.80 inches 2.22 pounds 37.6 cm 1005 grams
29 weeks 15.2 inches 2.54 pounds 38.6 cm 1153 grams
30 weeks 15.71 inches 2.91 pounds 39.9 cm 1319 grams
31 weeks 16.18 inches 3.31 pounds 41.1 cm 1502 grams
32 weeks 16.69 inches 3.75 pounds 42.4 cm 1702 grams
33 weeks 17.20 inches 4.23 pounds 43.7 cm 1918 grams
34 weeks 17.72 inches 4.73 pounds 45 cm 2146 grams
35 weeks 18.19 inches 5.25 pounds 46.2 cm 2383 grams
36 weeks 18.66 inches 5.78 pounds 47.4 cm 2622 grams
37 weeks 19.13 inches 6.30 pounds 48.6 cm 2859 grams
38 weeks 19.61 inches 6.80 pounds 49.8 cm 3083 grams
39 weeks 19.96 inches 7.25 pounds 50.7 cm 3288 grams
40 weeks 20.16 inches 7.63 pounds 51.2 cm 3462 grams
41 weeks 20.35 inches 7.93 pounds 51.7 cm 3597 grams
42 weeks 20.28 inches 8.12 pounds 51.5 cm 3685 grams

Where to go next:

Fetal ultrasound measurements show how the baby is growing and also help detect abnormalities. The estimation of fetal weight during pregnancy is among the most important examinations done. These measurements may help your doctor determine whether the baby is too small (intrauterine growth restriction: IUGR) or too big (large for gestational age: LGA).

Babies that are too small or too large have higher risks of complications. Because the early detection of growth abnormalities may help to manage complications more appropriately even before the baby is born, monitoring fetal growth is an important part of antepartum care.

Monitoring can be done through several steps including palpating the uterus and the fetus, measuring the size of the uterus, and performing a sonogram. A sonogram will measure various parts of the fetus, including the head, abdomen, and upper thighbone.

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