Fetal Development
Alternative names:
blastocyte; embryo; morula; zygote
Information:
Definition:
The development of the fetus from the time of coitus and conception to birth.
Once sperm is deposited in the vagina, it travels through the cervix and into the Fallopian tubes. Conception usually takes place in the outer third of the Fallopian tube. A single sperm penetrates that egg and a joining of the genetic information occurs. This resulting single cell is called a zygote.
The zygote spends the next few days traveling down the Fallopian tube and rapidly multiplying the number of cells through division. A ball of cells results from the cell division, each with a copy of the genes that will become the fetus. This ball of cells in the Fallopian tube is called a morula.
With additional cell division, the morula becomes an outer shell of cells with an attached inner group of cells. This stage of embryonic development is called a blastocyte. The outer group of cells become the membranes that nourish and protect the inner group of cells which becomes that fetus.
The blastocyte continues the journey down the Fallopian tube and between the 7th and 9th day after conception, implants in the uterus. At this point in the mother's menstrual cycle the endometrium (the lining of the uterus) has grown, is highly vascular, and ready to support a fetus. The blastocyte burrows into the endometrium where it receives nourishment. The placenta and supporting infrastructure for pregnancy grow out of the implantation phase. It is estimated that up to 55% of zygotes never reach this phase of growth.
It is not uncommon when there is a problem with embryonic or fetal development to have problems with other tissues that developed at the same time. For example if a child has developmental kidney problems, it is possible that the child may have hearing problems as well since those body organs develop at the same time.
The embryonic stage begins on the 15th day after conception and continues until about the 8th week, or until the embryo is 1.2 inches in length. During this period the cells of the embryo are not only multiplying, but they are taking on specific functions. This process is called tissue differentiation and is necessary for the different cell types that make up a human being (such as blood cells, kidney cells, nerve cells, and so forth). There is rapid growth and the main external features begin to take form. It is during this period that the growing child is most susceptible to teratogens (substances that cause birth defects).
Specific changes by week for the embryonic stage:- week 3
- formation of the heart
- beginning development of the brain and spinal cord
- beginning of the gastrointestinal tract
- teratogens introduced during this period may cause
- absence of one or more limbs
- a heart that is outside of the chest cavity
- weeks 4 to 5
- formation of the tissue that develops into the vertebra
- formation of the tissue that develops into the lower jaw, hyoid bone, and the cartilage of the larynx
- beginning of the structures of the ear
- further development of the heart which now beats at a regular rhythm
- movement of rudimentary blood through the main vessels
- beginning of the structures of the eye
- the brain develops into five areas and some cranial nerves are visible
- arm and leg buds are visible with hand and foot pads
- teratogens introduced during this period may cause
- omphalocele
- transesophageal fistulas
- hemivertebra
- nuclear cataract
- abnormally small eyes
- facial clefts
- absence of hands or feet
- week 6
- formation of the nose
- further development of the brain
- straighter posture
- upper and lower jaws are recognizable
- the trachea (windpipe) is developed with two lung buds
- upper lip is formed
- palate is forming
- ears are developing
- arms and legs have lengthened with arms more developed than legs
- hands and feet have digits, but may still be webbed
- tail is receding
- heart is almost fully developed
- fetal circulation is more developed
- teratogens introduced during this period may cause
- profound heart or aorta abnormalities
- cleft lip
- absence of lower jaw
- week 7
- head is more rounded
- eyes move forward on the face
- eyelids begin to form
- the palate is nearing completion
- tongue begins to form
- gastrointestinal tract separates from the genitourinary tract
- all essential organs have, at least, begun to form
- teratogens introduced during this period may cause
- week 8
- the embryo now resembles a human
- facial features continue to develop
- the eyelids start to fuse
- the external features of the ear begin to take their final shape, but are still set low on the head
- beginnings of external genitalia form
- anal passage opens, but the rectal membrane is intact
- circulation through the umbilical cord is well developed
- long bones begin to form
- muscles are able to contract
- teratogens introduced during this period may cause
- persistent opening in the atrium of the heart
- stunting of the digits
At this point the embryo is developed enough to call a fetus. All organs and structures found in a full-term newborn are present. The period of fetal development is a time of growth and further development of those structures that differentiated during the embryonic period.
- weeks 9 to 12
- the fetus reaches a length of 3.2 inches
- the head comprises nearly half of the fetus' size
- a neck is present
- the face is well formed
- the ears begin to have a fully developed appearance
- eyelids close and will not reopen until about the 28th week
- tooth buds appear for the baby teeth
- limbs are long and thin
- digits are well formed
- the fetus can make a fist with its fingers
- urogenital tract completes development
- genitals appear well differentiated
- red blood cells are produced in the liver
- fetal heart tones are heard with electronic devices
- weeks 13 to 16
- the fetus reaches a length of about 6 inches
- a fine hair develops on the head called lanugo
- fetal skin is almost transparent
- more muscle tissue and bones have developed and the bones become harder
- the fetus makes active movements
- sucking motions are made with the mouth
- amniotic fluid is swallowed
- meconium is made in the intestinal tract
- lungs further develop
- sweat glands develop
- the liver and pancreas produce their appropriate fluid secretions
- week 20
- the fetus reaches a length of 8 inches
- lanugo hair covers entire body
- skin becomes less transparent as fat begins to deposit
- nipples appear
- eyebrows and lashes appear
- nails appear on fingers and toes
- the fetus is more active with increased muscle development
- the quickening usually occurs (where the mother feels the fetus moving)
- fetal heartbeat can be heard with a stethoscope
- week 24
- the fetus reaches a length of 11.2 inches
- the fetus weighs about 1 lb. 10 oz.
- hair on the head is longer
- eyebrows and eyelashes are well formed
- all the eye components are developed
- the fetus has a hand and startle reflex
- thicker skin on hands and feet
- footprints and fingerprints forming
- alveoli forming in lungs
- entire body covered in vernix caseosa (a protective cheese-like substance secreted by the fetus)
- weeks 25 to 28
- the fetus reaches a length of 15 inches
- the fetus weight about 2 lbs. 11 oz.
- rapid brain development
- nervous system developed enough to control some body functions
- eyelids open and close
- testis begin descent into the scrotal sack if the fetus is male
- respiratory system, while immature, has developed to the point where gas exchange is possible
- a baby born at this time may survive, but the possibilities for complications and death remain high
- weeks 29 to 32
- the fetus reaches a length of about 15-17 inches
- the fetus weights about 4 lbs. 6 oz.
- rapid increase in the amount of body fat
- increased central nervous system (CNS) control over body functions
- rhythmic breathing movements present
- lungs are not fully mature
- fetal body temperature partially self-controlled
- bones are fully developed, but still soft and pliable
- male testis still in inguinal canal
- fetus begins storing iron, calcium, and phosphorus
- week 36
- the fetus reaches a length of about 16-19 inches
- the fetus weighs about 5 lbs. 12 oz. to 6 lbs. 12 oz.
- lanugo begins to disappear
- increase in body fat
- fingernails reach the end of the fingertips
- increased central nervous system (CNS) control over body functions
- a baby born at 36 weeks has a high chance of survival, but may require some interventions
- weeks 38 to 40
- considered full-term at 38 weeks
- may be 19 to 21 inches in length
- males typically weigh more than females
- lanugo is gone except for on the upper arms and shoulders
- fingernails extend beyond fingertips
- small breast buds are present on both sexes
- head hair is now coarse and thicker
- mother supplies fetus with antibodies against disease
- fetus fills entire uterus
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The information provided herein should not be used for diagnosis or treatment of any medical condition. A licensed physician should be consulted for diagnosis and treatment of any and all medical conditions. Copyright 2000 adam.com, Inc. Any duplication or distribution of the information contained herein is strictly prohibited.
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