Calculators & Apps

The 6 country meta-analysis sex-specific percentiles, which were used to develop the Fenton Preterm Growth Charts, can be used to assign size for gestational age (small, large and appropriate) at birth. This data is made freely available under an open Creative Commons License for non-commercial use. 

Z-scores are:

1. Similar to percentiles but even more useful than percentiles, Z-scores are the number of standard deviations above or below a growth chart’s center (mean or median) curve, so they are a way to define an infant’s placement on a growth chart for a given age. Z-scores are better than percentiles since percentiles are not spaced evenly at the highest and lowest values while Z-scores are evenly spaced for any value.
2. Changes in Z-scores can tell if a second measurement is closer or further away from the growth chart center than the previous measurement. Changes in Z-scores from birth are not useful to quantify “growth” since the post-natal weight loss phase is included, which is not a growth phase, and expert groups do not recommend that infants gain to regain their birth percentile. Z-score changes in size () can be quantified for research purposes, but they should be referred to as changes in z-scores and be not referred to as growth.

To quantify growth using Z-scores:

It is better to begin quantifying growth at the weight nadir [usually 2 to 7 days of age] or at day 7 (1). Short time periods will show excess variability (3) so could lead to frequent changes in care analogous to micromanagement. It is better to use a minimum of 5 to 7 days to quantify growth.

Expectations about changes in Z-scores:

Goldberg and colleagues recommend working to try to avoid losses of greater than 2Z-scores from birth (3); this recommendation requires validation. Changes in Z-scores can be used for weight, length and head circumference. Rochow and colleagues quantified that among the healthiest of preterm babies from healthy pregnancies lost an average of 0.8 Z-scores in the first three weeks of life, but the losses of Z-scores ranged from a loss of 2 Z-scores to no loss, among this very healthy cohort (4).

It is rare for preterm infants to regain their birthweight percentiles by or before 40 weeks post menstrual age; infants who are growth restricted infants are more likely than non- growth restricted infants to regain their birthweight percentiles (5,6), but regaining birthweight percentiles is not common for any category of infants.

Use only if age is defined in both weeks and days

Uses Completed weeks

for weight, Head circumference, and Length - can calculate up to 500 babies' completed weeks z-scores and exact percentiles at one time.

Many thanks to Dr. Tim Stevens for his assistance in preparation of these spreadsheets. 
These results differ from the Actual Age Calculator since they are based on Completed weeks (24 weeks and 0 to 6 days are categorized as 24 weeks), which is usually of utility for research purposes. 

Do not enter days of age into this calculator as it causes this calculator to incorrectly round up or down

A Web-based calculator is available here:

1. Fenton TR, Griffin IJ, Hoyos A, Groh-Wargo S, Anderson D, Ehrenkranz RA, Senterre T. Accuracy of preterm infant weight gain velocity calculations vary depending on method used and infant age at time of measurement. Pediatr Res. 2019 Apr;85(5):650-654. PMID: 30705399.
2. Fenton TR, Senterre T, Griffin IJ. Time interval for preterm infant weight gain velocity calculation precision. Arch Dis Child Fetal Neonatal Ed. 2019 Mar;104(2):F218-F219. PMID: 29997166.
3. Goldberg DL, Becker PJ, Brigham K, Carlson S, Fleck L, Gollins L, Sandrock M, Fullmer M, Van Poots HA. Identifying Malnutrition in Preterm and Neonatal Populations: Recommended Indicators. J Acad Nutr Diet. 2018 Sep;118(9):1571-1582. PMID: 29398569
4. Rochow N, Raja P, Liu K, Fenton T, Landau-Crangle E, Göttler S, Jahn A, Lee S, Seigel S, Campbell D, Heckmann M, Pöschl J, Fusch C. Physiological adjustment to postnatal growth trajectories in healthy preterm infants. Pediatr Res. 2016 Jun;79(6):870-9. PMID: 26859363.
5. Molony CL, Hiscock R, Kaufman J, Keenan E, Hastie R, Brownfoot FC. Growth trajectory of preterm small-for-gestational-age neonates. J Matern Fetal Neonatal Med. 2021 Sep 9:1-7. PMID: 34503371
6. McLaughlin EJ, Hiscock RJ, Robinson AJ, Hui L, Tong S, Dane KM, Middleton AL, Walker SP, MacDonald TM. Appropriate-for-gestational-age infants who exhibit reduced antenatal growth velocity display postnatal catch-up growth. PLoS One. 2020 Sep 8;15(9):e0238700. PMID: 32898169