FoundationsGrowth and organ differentiation.
The fetal period is about continued differentiation of organs and tissues, most importantly this period is about growth both in size and weight.
Human Fetal Development cartoon (not to scale)
(More? Human Development cartoon (on new page, 800 Kb)
The cartoon above shows changes in the relative proportions of head and body and growth of the limbs, it does not show the overall growth in size.
Many different systems formed in the embryonic period (organogensesis) grow and differentiate further during the fetal period and do so at different times.
For example, the brain continues to grow and develop extensively during this period (and postnatally), the respiratory system differentiates (and completes only just before birth), the urogenital system further differentiates between male/female, endocrine and gastrointestinal tract begins to function.
Finally consider the systems (for example respiratory, cardiac, neural) that will still not have their final organization and function determined until after birth.
This is an enlarged image of the actual size comparison shown in the introduction of the Embryonic stage 13, 23 and Fetal stage 10 week 40mm.
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.
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 weights (macrosomia) can be an indicator of unregulated maternal diabetes.
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)
This page is a link to images from a 10 week female fetus approximately 40 mm in size.
This stage of development is after the embryonic period (up to week 8) but still only 2 weeks into early fetal development.
There are 4 sections taken in the parasagittal and sagittal plane (moving from the right at Plane A towards the midline at Plane D). Click on the small images (or the text below) to open the linked large image pages.
10 Week Plane A | 10 Week Plane B | 10 Week Plane C | 10 Week Plane D
(More? Week by Week | human embryology movies)
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Link to next page in this Practical - Neonatal Period This is all about the Neonatal Period of development. |
The fetal period is about growth and ongoing organ development. Think about what organs and tissues are required for this process and which develop last.
At the end of the fetal period the birth process (parturition) occurs, a process initiated by the fetus.
At birth there are many changes that must occur to survive in the external environment.
These notes and linked materials have been prepared for Educational purposes only.
Please email Dr Mark Hill if you wish to make a comment about this current project.
Use the alphabetical list below to find definitions of terms that are new to you.
A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W |X | Y | Z |
fetal length - The measurement of crown to rump length of the developing fetus. The greatest growth in length occurs in the middle second trimester, of human development. There are a number of other growth parameters that can be measured, commonly determined by ultrasound, during the fetal period. (More? Fetal Length | Fetal Notes | Ultrasound Measurements | Ultrasound Movies)
fetal period - In humans, the development week 9 to 36 is the fetal stage (second and third trimester) and during this time organs formed in the embryonic period continue to develop and the fetus grows in size and weight. The first 8 weeks of development is considered the embryonic period and is divided into 23 Carnegie stages based upon developmental milestones. Note when searching an alternate spelling "foetal". (More? Fetal | Embryo Stages)
fetal weight - The measurement of the weight of the developing fetus. The measurement is obtained by ultrasound calculation or clinically estimated by palpatation. The greatest addition of fetal weight occurs during and towards the end of the third trimester. (More? Fetal Weight | Fetal | Ultrasound Measurements | Ultrasound Movies)
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 | Human Fetal Weight | 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 Length | Fetal Weight | Fetal Notes)
ultrasound - A noninvasive technique for visualizing the follicles in the ovaries and the gestational sac or fetus in the uterus. Uses high-frequency sound waves that are reflected off internal structures. These reflections can be analysed and displayed by computer. (More?Ultrasound Measurements | Ultrasound Movies | Abnormal Ultrasound Movies)
very low birth weight - (VLBW) This is defined as weight at birth of less than 1500 grams (3 lb, 5 oz). (More? Human Fetal Weight | Abnormal Notes IUGR)
Movie of Human Embryo Growth (this shows a human embryo growing, all images are to scale)
Movie of Mouse Embryo Growth (this shows a mouse embryo growing)
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