«This document describes the application of the customised growth potential to assess fetal size and growth, using the Gestation Related Optimal ...»
Updated July 2015
This document describes the application of the customised growth potential to assess fetal size
and growth, using the Gestation Related Optimal Weight (GROW) software.
GROW – Customised Weight Centiles
- to calculate birthweight centiles individually or in bulk for whole databases;
GROW – Customised Growth Charts
- to plot fundal height and estimated fetal weight.
2. General concepts
3. Calculating the Optimal Weight
4. Coefficients for adjusting the Term Optimal Weight (TOW)
5. Proportionality curve
6. Normal range GROW – Customised Weight Centiles 7.
GROW – Customised Growth Charts 8.
GRAW – Population Average Growth Charts 9.
10. Gestational age calculation
1. Introduction Back to Contents The customised growth chart concept was developed initially in Nottingham in the early 1990s 1.
While recognising the importance of growth for fetal well being, we became increasingly aware that existing charts were not useful for clinical assessment in a heterogeneous maternity population.
Over time, we have been able to test the concept of adjustable or customised assessment of growth and birthweight from many different perspectives. We are constantly seeking to improve and add to the database which allows application in different populations.
The project has been fortunate to benefit from a number of dedicated researchers, statisticians and programmers over the years, who are acknowledged in various publications referenced here.
I would particularly like to acknowledge the ongoing help of Andre Francis, statistician and long time friend. But while the strengths of the method, supporting evidence and implementation are thanks to the efforts of my collaborators, any weaknesses are entirely my own responsibility.
We hope that you find our software useful for the assessment of fetal growth and birth weight. We are continuing to seek to improve it, and comments and criticisms are always welcome, so please do not hesitate to get in touch!
Professor Jason Gardosi MD FRCSED FRCOG Director, Perinatal Institute, Birmingham B15 3BU, United Kingdom Tel +44 121 607 0101; Fax +44 121 607 0102 e-mail: email@example.com;
websites: www.perinatal.org.uk and www.gestation.net Referencing: The software can be referenced as follows, with the relevant version (‘x’) of the
Birthweight or fetal weight centiles:
Gardosi J, Francis A. Customised Weight Centile Calculator. GROW version x, Year.
Gestation Network, www.gestation.net
Antenatal growth charts:
Gardosi J, Francis A. Customised Antenatal Growth Chart - GROW version x, Year.
Gestation Network, www.gestation.net Disclaimer: GROW software makes calculations on the basis of our published principles and formulae, and is released only after rigorous testing and strict quality assurance. However the Gestation Network and the Perinatal Institute cannot accept responsibility for clinical diagnoses or decisions made on the basis of the software. For more information, please see our terms and conditions of use.
Copyright and Trademark: To ensure quality control in the software and its application, the use and display of the principles we developed and are describing here (term optimal weight, proportionality curve etc) is protected by copyright © Gestation Network, and GROW™ and GROW-App™ are trademarked. Fur further information, please contact firstname.lastname@example.org
2. General concepts Back to ContentsThe software allows the generation of an individual or ‘customised’ standard by adjusting for physiological factors which are known to affect fetal growth. The pregnancy characteristics are entered to calculate the Term Optimal Weight (TOW, Section 3). This is the weight that the baby is predicted to achieve in the absence of pathological influences. The calculation of TOW is centred on 40.0 weeks (280 days).
Through this point TOW, the proportionality curve is plotted to delineate how this weight is expected to be reached in a normal pregnancy (see section 5). This gives an individually adjusted Gestation Related Optimal Weight (GROW) curve. Around this optimal line, the normal variation can be calculated and limits such as the 10th and 90th centile lines drawn. Thus neonatal weights from previous pregnancies, as well as fundal height measurements or fetal weight estimations in the current pregnancy, can be seen in relation to individually adjusted optimal weight limits.
There are 3 underlying principles for GROW-percentiles:
1. Weights are assessed in reference to a standard that is individually adjusted for physiological pregnancy variables (maternal height, weight, parity and ethnic group); e.g. at 40 weeks, a 3000g baby is small for an average size mother but may be normal for a small mother. Postnatally, the standard is also adjusted for the sex of the baby.
2. The standard is ‘optimised’ to obtain the growth potential, i.e. pathological variables such as smoking and diabetes are excluded. For example, expected term baby weight for a mother who smokes is calculated as if she was a non-smoker so that, if her baby’s growth is affected, the growth or weight deficit is more likely to be identified.
3. Optimal weight is calculated using a fetal rather than a neonatal weight standard. Preterm neonatal weights are abnormal by definition, and have often been affected by fetal growth restriction preceding spontaneous or iatrogenic preterm delivery. Eg. at 32 weeks, a 1500g baby would fall within normal birth weight limits, but is small according to a 32 week fetal weight standard derived from pregnancies which have gone on to normal delivery at term.
3. Calculating the Optimal Weight Back to ContentsThe main non-pathological factors affecting birth weight are gestational age, maternal height, maternal weight at booking, parity and ethnic group.1 The sex of fetus/neonate, when known, can also be adjusted for. Coefficients to adjust for these variables were originally derived from a dataset of around 30,000 ultrasound dated deliveries. They allow calculation of an expected birth weight for each pregnancy, and the ‘customised’ percentile which a particular weight has achieved in relation to this expected endpoint.
An alternative method to adjust for such variables is to calculate the individual birth weight ratio (IBR). 2 3 4. IBR follows principle 1 above, i.e. adjusts for individual variation, but does not optimise (principle 2) or apply a fetal weight standard (principle 3).
In pre-2009 versions, adjustment for maternal weight was made within BMI limits of 20-30 only.
Since then, evidence has shown that the association between SGA babies and perinatal mortality is stronger without these limits. However, in current versions of the software, adjustment is made only to exclude extreme values, defined as outside the BMI range of 15-50.
Other physiological variables such as paternal height have, unless extreme, a relatively minor effect 5 and may in any case not always be known with certainty. Maternal age usually appears to play no significant role once parity is adjusted for.
Pathological factors such as smoking, social deprivation, pre-eclampsia or diabetes are also known to be related to birth weight but are not adjusted for. The purpose is to calculate the ‘term optimal weight’ (TOW) as an ideal standard, against which the actual fetal or neonatal weight can be assessed. Risk factors should be used as clinical prompts to increase surveillance. TOW is centred on day 280, the median and modal length of pregnancy in our population 6.
4. Coefficients for adjusting the term optimal weight (TOW) ContentsCoefficients are derived from suitable databases using a multiple regression model centred on a standard gestational age (280 days), the largest ethnic group, average maternal height and weight at booking, and first pregnancy (para 0). In addition, gender is listed as an ‘average’ i.e. sexneutral. The regression model has a constant to which weight is added or subtracted for each of the variables, according to the formula TOW = constant + htao + wtao + ethao + parao + sexao where ‘ao’ are add-ons, respectively, for ht = maternal height wt = maternal weight at booking (first visit) eth = ethnic origin par = parity and sex = gender of fetus/neonate, if known Coefficients The original coefficients were derived from a Nottingham database (1987-1991; n=30,000) 6 and are reproduced here for illustration (Please note – these coefficients have since been superseded).
Since then, sets of coefficients have been developed from more recent and more detailed databases in UK and other countries, including Australia, New Zealand, United States, Ireland, Netherlands, Spain, Sweden, Brazil, and incorporated into country specific versions of GROW.
Additional / updated sets of coefficients are being developed constantly as data become available.
5. Proportionality curve Back to ContentsOnce the TOW (term optimal weight, predicted for 280 days gestation) is calculated, it is combined with a proportionality growth function to determine the optimal weight at all gestations. This function transforms the average weights at all gestations to a percent of term weight in that population. The proportionality principle can be used retrospectively (birthweight to fetal weight) or to project fetal weight to predict birth weight 6 7.
Reviews of published formulae for fetal weight gain suggest that most follow a similar pattern, or growth dynamic, although the endpoints (term weights) may vary 8 9. Our standard formula is derived from Hadlock’s fetal weight equation 10 which closely reflects normal fetal weight in other
populations. The proportionality equation is :
% weight = -5.86048381 + 1.419180433 GA - 0.116517911 GA2 + 0.004154453 GA3
where GA is gestational age between 20 and 42 weeks.
Thus for each individually predicted Term Optimal Weight (TOW), the formula is used to produce a Gestation Related Optimal Weight (GROW).
It should be noted that, owing to the techniques used in its derivation, the above proportionality equation yields 100.3% when GA=40.
The normal limits of weight for all gestations are calculated from the coefficient of variation (CV) of the TOW. It is derived from the SD and Mean (Constant) of the population through the regression
model, and defined as:
For the original UK database, SD = 389, Constant = 3478. Thus, in this case CV = 11% The centile limits are derived using Z scores. For example, the 90th and 10th centiles are represented by z = 1.28.
E.g. the 10th to 90th centile range for a TOW of 3500g is 3500 14%, i.e. range 3010-3990g.
The effect of using the CV is that the range designated as ‘normal’ becomes narrower for lower TOWs and wider for higher TOWs. Thus a small baby is allowed a smaller range of normal variation in absolute terms. The method compensates for the positive skewness of the distribution of birth weight.
The proportionality weight equation is fitted through the three term points: TOW, TOW+14% and TOW–14%. This defines the 50th, 90th and 10th centile lines respectively for the gestation period 24 to 42 weeks. This principle is used in the applications described in the following sections.
This module allows a customised weight-for-gestational age centile to be determined for previous babies, and for estimated fetal weights and birth weight in the current pregnancy.
The calculators are provided in two forms:
Individual Centile Calculator (ICC), on-line for case-by-case use, and Bulk Centile Calculator (BCC), in spreadsheet format to allow calculation for whole databases.
Precise gestational age (at birth, or at the point of EFW measurement) needs to be entered.
Gestational age can be calculated with the ‘Gestational Age Calculator’ - see section 10 below.
The applications can also be used for an estimated fetal weight (EFW) centile when the sex of the baby is not known. When other variables are missing or unobtainable - e.g. maternal height partial customisation can be undertaken by entering an estimate or population average. In the BCC, this is done automatically when a variable is omitted.
Bulk Centile Calculator versions and history
The centile calculator is also integral to the customised growth chart, to calculate centiles from previous pregnancies as they may be relevant for care in the current pregnancy. The centiles are calculated for the corresponding parity of the mother at the beginning of the respective pregnancy.
NB: No adjustment is made for maternal weight if it was different in a previous pregnancy; if her weight is considered to have been significantly different, we recommend that previous birthweight centiles are calculated for the respective maternal variables separately.
8. GROW – Customised Growth Charts Back to ContentsThe GROW Customised Growth Chart module allows the generation of antenatal charts After entering the pregnancy data through the 'Mother details', 'Baby details' and 'EDD' sub-routines, the chart is generated on screen and can be printed out in early pregnancy.
It shows a summary of the pregnancy details and the BMI calculated from maternal height and weight previous babies’ birthweight centiles
The relationship between weight and fundal height is described by a formula derived from a study of 325 pregnancies11, showing a relationship in the third trimester of the form ln(EFW) = 10.6857 – 100.25/SFH where EFW is in grams and SFH is in cms.