The fetal period (9-36 weeks) is about continued differentiation of organs and tissues, most importantly this period is about growth both in size and weight.
The measurement of the weight of the developing fetus. The measurement is obtained by ultrasound calculation (sonographic fetal weight estimate) or clinically estimated by palpatation.
The greatest addition of fetal weight occurs during and towards the end of the third trimester, just before birth.
Fetal origins hypothesis identifies maternal derived abnormalities relate to lifestyle, environment and nutrition and while some of these directly effect embryonic development, there is also growing evidence that some effects are more subtle and relate to later life health events, originally called the "Barker Hypothesis". (More? Fetal Origins or Fetal Programming Hypothesis)
Fetal weight estimation by ultrasound: comparison of 11 different formulae and examiners with differing skill levels. Siemer J, Egger N, Hart N, Meurer B, Müller A, Dathe O, Goecke T, Schild RL. Ultraschall Med. 2008 Apr;29(2):159-64. Epub 2007 Jun 29. PMID: 17602369
"Some regression formulae appear to be favourable within defined weight ranges. Accuracy of the formulae, however, is still unsatisfactory, and new formulae focusing on specific weight ranges (e. g., macrosomic fetuses) are needed. In addition, experience in obstetric ultrasound improves accuracy of fetal weight estimation."
Newborn weight charts underestimate the incidence of low birthweight in preterm infants. Burkhardt T, Schäffer L, Zimmermann R, Kurmanavicius J. Am J Obstet Gynecol. 2008 Aug;199(2):139.e1-6. Epub 2008 Apr 18. PMID: 18395687
"The third centiles of sonographic and newborn weights diverged markedly between 25 and 36 weeks of gestation and by more than 400 g at 32-33 weeks. In contrast, sonographic and newborn PI values were similar despite uncertainties as to fetal length."
Low Birth Weight
Low birth weights can be an indicator of intrauterine growth restriction or retardation (IUGR), where the fetus that has not reached its growth potential because of genetic or environmental factors (less than 10th percentile for gestational age)
High Birth Weight
High birth weights (macrosomia) can be an indicator of unregulated maternal diabetes.
Graph shows average Human Fetal Growth from Week 9 to 36. Weight is measured in grams, Length is crown/rump in millimeters.
During the fetal period there is a separation in changes in length and weight.
Fetal length change is greatest in the middle period (second trimester).
Fetal weight change is greatest in the final weeks of development (third trimester).
To open the graphs separately and alone on a new page click link below.
In addition to these obvious maternally-derived abnormalities, there is growing evidence that the interuterine environment has a strong influence on later postnatal health. This theory is based on the early statistical analysis of disease/longevity in babies with low birth weights in England by Barker, and has been called the "Barker Hypothesis".
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intrauterine growth restriction - (IUGR) Term used to descibe clinically a fetus that has not reached its growth potential because of genetic or environmental factors. Abnormal development measured as less than 10th percentile for gestational age, not easy to detect before 32 weeks. This poor fetal growth can have fetal, placental or maternal causes. (More?Abnormal Notes IUGR | Fetal Length | Fetal Notes | Fetal Programming Hypothesis | Medline Plus - IUGR)
macrosomia - Term used to describe a newborn with an excessive birth weight. The definition is either a birth weight of 4000 to 4500 g (8 lb 13 oz to 9 lb 15 oz) or greater than 90% for gestational age after correcting for neonatal sex and ethnicity. (More? Human Fetal Weight | Maternal Diabetes)
ponderal index - (PI) - Fetal calculation based upon ratio of body weight to length PI = [weight (in g) x 100] ÷ [length (in cm)] (More? | Fetal Notes)