STAGES OF PRENATAL DEVELOPMENT College of William and Mary Students for Life 2013
Germinal Period From conception to implantation
Terms to Know Conception: the first stage in human development in which a sperm cell meets with an egg cell Zygote: the single cell formed at conception from the union of the sperm and the ova Morula: the fertilized ovum once it has begun to multiply into many cells but before implantation
Fertilization A Unique Human When the sperm meets with the egg, a new human, with a DNA structure unique from that of either parent is formed The zygote may have a different blood type than the mother This is not a small or arbitrary difference If the mother has a negative blood type and the zygote has a positive blood type, the mother s body will fight against the foreign blood, which may cause a miscarriage (in order to ensure that this does not happen, the woman must receive regular injections before conception occurs) If the zygote were simply another part of the woman s body, it would necessarily be more similar in structure to her DNA and blood
The Rest of the Germinal Period In total, the germinal period lasts for 8 days (ending well before a woman knows that she is pregnant); she has most likely not yet missed a period Once the morula attaches to the uterine wall, it is considered an embryo and the germinal period ends This would be the clump of cells appearance that many abortion clinics may refer to; however, this stage ends long before a woman would begin to seek an abortion
The Embryonic Period From implantation to approximately eight weeks
Terms to Know Cephalocaudal: the body develops from the head down Proximodistal: the body develops from the inside out Gonadal ridges: where sex organs will eventually develop Embryo: the stage of human development from implantation until all organs begin to develop (roughly 8 weeks)
Development of the Embryo* Development is both cephalocaudal and proximodistal; this means that the central nervous system and the heart are the first things to begin developing 3 weeks after conception The first heartbeat occurs (despite the heart not being fully formed) Three major divisions of the brain begin to differentiate (hindbrain, midbrain, and forebrain) 5 weeks after conception: bronchial buds (which will become lungs) are formed 6 weeks after conception: Lower jawbone fuses and upper jawbone begins to form The external ear begins to form The three main parts of the brain are distinct 7 weeks after conception: Although the sex of the embryo has been decided since conception (when the sperm donated either a second X chromosome female or a Y chromosome male), it is only now that the distinct physical differentiation begins to take place and gonadal ridges begin to form Some people seem to believe that it is only at this point that the embryo has a sex, but this is incorrect! They have had the chromosomal make-up to become either a male or female since conception, it is only now that those physical characteristics take form Muscles begin to assume their final shapes and relationships The brain develops thousands of nerve cells per minute 8 weeks after conception: The external, middle, and inner ears assume their final form the embryo can hear its mother s voice and the sounds of its environment By the end of this week, the fetus is able to move around slightly and responds to stimulation around its mouth *These are just the major developments; each week has many more milestones not compiled here
The Fetal Period From approximately eight weeks to birth
Terms to Know Fetus: the stage in human development from approximately 8 weeks until birth Retroflexion: bending, curving, or turning backward Anteflexion: bending, curving, or turning forward
Milestones of the Fetal Period The fetal period officially begins when all the basic tissues and organs are in place in a rudimentary form and when the skeleton begins to ossify 10 weeks after conception: intestines are in place and the fetus has the ability to breathe 12 weeks after conception: Sexual characteristics fully defined Sucking and swallowing movement 20 weeks (5 months) after conception: fetus has almost as many nerve cells as it will ever have 24 weeks (6 months) after conception: olfactory cells are functional and fetus has the ability to smell (only minutes after birth, an infant will show preference for the smell of its mother and for the smell of its own amniotic fluid) 28 weeks (7 months) after conception: lungs fully formed Over the course of the fetal period, all organs continue to grow and develop
Sensory Capacities Motion At 15 weeks, the fetus is capable of all of the movements observable in a newborn infant At 20 weeks (5 months), the fetus middle ear (controlling its sense of balance) begins to develop this means that it can sense changes in its mother s posture and orient itself accordingly Breathing in a fetus is, for obvious reasons, different than in an infant; however, it makes movements with its chest and lungs that simulate breathing and will allow it to breathe once outside of the womb Movement Any movement Startle response Generalized movements Hiccups Isolated arm movements Head retroflexion Hand-face contact Breathing Jaw opening Stretching Head anteflexion Yawn Suck and swallow Gestational Age 7 weeks 8 weeks 8 weeks 8 weeks 9 weeks 9 weeks 10 weeks 10 weeks 10 weeks 10 weeks 10 weeks 11 weeks 12 weeks
Sensory Capacities (Cont d) Sight At 26 weeks, if a bright light is held against the mother s abdomen, the fetus may respond with changes in heart rate and movement It has been postulated that a fetus sees light similarly to the way a flashlight looks when covered by a hand It is important to note that the slow development of sight in a fetus is not something that differentiates it significantly from a newborn; a newborn can only see approximately one foot in front of it; farther than that, the infant can only distinguish shadows and basic shapes
Sensory Capacities (Cont d) Hearing Experimental Evidence 20-24 weeks (5-6 months) after conception: the fetus responds to sounds 24-28 weeks (6-7 months) after conception: the fetus can recognize changes in sounds (begins to differentiate the sounds present in the language its mother speaks from the sounds present in other languages) The sound level of the outside world to a fetus is roughly equal to the level at which one would hear the outside world when sitting in a car with the windows up The mother s heartbeat and voice are able to be heard the best Language Acquisition Experiments Infants were played sounds from their own language as well as sounds only present in other languages The infant was then played two sounds, one of which was present in its native language and one of which was not; they responded more strongly to the novel sound, indicating that they recognized it was not in the language they were used to hearing In another study, fetuses still in the womb were shown to prefer sounds in their native language Evidence for Hearing in the Womb Infants were read two stories by their mothers, one that had been read aloud to them while they were in the womb and one they had never heard before The infants were then conditioned to be able to control what story would be read to them in a lab experiment (in one group, if they sucked faster, their mothers would read them the familiar story slower sucking would cause their mothers to read the unfamiliar story and in the other group, they had to suck slower to hear the familiar story and faster for the unfamiliar) The infants showed a clear preference for the familiar story and wanted their mothers to read it to them This indicates that they were not only able to hear the stories read to them in the womb, but remember them as well
Sensory Capacities (Cont d) Taste Experimental Evidence 7 weeks after conception: taste buds are functional Fetuses will increase their swallowing when sweet tastes are presented into the amniotic fluid and will decrease their swallowing when bitter and sour tastes are presented (this mimics the reaction of newborns) Half of a group of pregnant women were told to drink carrot juice while pregnant (the other half acted as a control and drank only water) When these women had their babies, the newborns were tested for taste preference between plain cereal and carrot-flavored cereal The infants of the women who drank carrot juice during their pregnancy were significantly more likely to prefer the carrot-flavored cereal than the infants of the women who only drank water This indicates that food preferences are formed in the womb
In Conclusion... The prenatal stage is simply one of many stages in the development of every human being. Note: How could you expect me to end with anything other than an adorable picture of a baby?