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.
Human Fetal Development cartoon (not to scale)
Fetal Sizes (between 3 to 5 months)
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. (More? Human Development cartoon on new page, 800 Kb)
Many different systems formed in the embryonic period (organogenesis) 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.
The Embryonic period involved transient structures to establish body and placental tissues, folding and form.
Fetal weight and length change during the second and third trimester
The long Fetal period (4x the embryonic period) is a time of extensive growth in size and mass as well as ongoing differentiation of organ systems established in the embryonic period.
Also consider the systems (respiratory, cardiac, neural) that will still not have their final organization and function determined until after birth.
Page Links: Introduction | Some Recent Findings | Studying Fetal Development | Reading | Embryo/Fetus Size Comparison | Fetal Growth (Weight/Length) | 10 Week Fetus | 12 Week Fetus | Maternal Derived Abnormalities | WWW Links | References | Glossary
Laifer-Narin S, Budorick NE, Simpson LL, Platt LD. Fetal magnetic resonance imaging: a review. Curr Opin Obstet Gynecol. 2007 Apr;19(2):151-156.
Alexander BT. Fetal programming of hypertension. Am J Physiol Regul Integr Comp Physiol. 2006 Jan;290(1):R1-R10.
Matsuoka T, Ahlberg PE, Kessaris N, Iannarelli P, Dennehy U, Richardson WD, McMahon AP, Koentges G. Neural crest origins of the neck and shoulder. Nature. 2005 Jul 21;436(7049):347-55.
Start with early Fetal development (week 10) by studying the structures and relative fetal proportions in comparison to what we saw in late embryonic development (stage 20, page 4 of all notes sections)
Then look at developmental abnormalities. While many genetically (and some maternal) based abnormalities will have had an impact on embryonic development, the Fetal period is potentially sensitive to maternal effects that impact upon interuterine growth. Along these lines there is growing evidence that low birth weight, and therefore inhibited fetal growth, may have postnatal effects on lifelong health outcomes (see Barker Hypothesis link below).
Finally look at the systems (for example respiratory, cardiac, neural) that will still not have their final organization and function determined until after birth.
Actual size comparison of Embryonic stage 13, 23 and Fetal stage 10 week 40mm.
An enlarged image of the same developmental stages Embryonic stage 13, 23 and Fetal stage 10 week 40mm.
This comparative image now shows the extensive growth in body length that occurs during the early fetal (second trimester) period.
Note also the change in relative proportions of the head to body ratios.
Both the upper and lower limbs also increase significantly in length.
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.
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.
The head undergoes two different forms of ossification (endochondral and intramembranous) in separate regions of the skull. Furthermore the two images (lateral and medial) identify regions of cartilage development. This page is a link to images from a 12 week fetus approximately 92 mm CRL in size.
The images are from Virginia Diewert and give clear examples of the ossification process occurring in the head during the early fetal period.
Relate to lifestyle, environment and nutrition. Some examples of this form of abnormality are the impact of excess alcohol on neural development (Fetal alcohol syndrome), viral infection (rubella) at a critical stage of development, inadequate dietry folate intake (neural tube defects), effects of prescription drugs (Thalidomide- limb development) and even maternal endocrine function (thyroid development).
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".
Search NCBI Bookshelf: fetal development
Search PubMed: Search July 2009 "fetal development" 376,853 reference articles of which 36,344 were reviews.