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End of embryonic periodFull Term 40 to 42 WeeksPregnancy Timeline by SemestersFetal PeriodEmbryonic DevelopmentEmbryonic DevelopmentFetal PeriodFetal PeriodFetal PeriodFetal PeriodFetal PeriodInfant PeriodFetal PeriodFetal PeriodFull Term White fat begins to be madeImmune system beginningBrain convolutions beginSensory brain waves begin to activateFetal liver is producing blood cellsHead may position into pelvisThe Appearance of SomitesBeginning Cerebral HemispheresA Four Chambered HeartFirst Detectable Brain WavesBasic Brain Structure in PlaceFetal sexual organs visibleBrown fat surrounds lymphatic systemInner Ear Bones HardenLungs begin to produce surfactantPeriod of rapid brain growthHead may position into pelvisFetal liver is producing blood cellsPregnancy Timeline by SemestersWhite fat begins to be madeImmune system beginningBrain convolutions beginSensory brain waves begin to activateBone marrow starts making blood cellsFinger and toe prints appearHeartbeat can be detectedEnd of embryonic periodFirst Thin Layer of Skin AppearsFemale Reproductive SystemOocyte Fertilized
The following 3 periods reflect more precisely than "trimesters" the nature of fetal developmental and maternal physical changes observed in pregnancy:
Visible Embryo Pregnancy Calculator
embryo period
From fertilization to the eigth week of pregnancy, the 23 Carnegie stages of human development pinpoint changes in this extraordinary period - ending when all internal and external structures necessary for fetal growth are present.


week 1 - fertilization,
cleavage (first cell division)
blastocyst implants in uterus

2.5 weeks - primitive streak, neural groove, somites appear

3.5 weeks - heart tube forms

4 weeks - body cavity forms

5 weeks -eyes, ears, nostrils, mouth begin

6 weeks - pituitary - intestines begin

7 weeks - kidneys produce urine

8 weeks - head erect and rounded; limbs recognizable

fetal period
From weeks 9 to 36 , the embryo begins fetal development. All organs systems are becoming viable - able to interact and eventually sustain life independent of the mother.

the eyes move from the sides of the head to the front

the bridge of the nose develops along with other forms of cartilage

hairs appear on the eyebrows and lips followed by fine lanugo hair covering the entire body

the umbilical cord herniates and this "gut" enters the body cavity

external genitals identify the sexes

nails form

the mouth opens and the tongue appears

breathing motion begins as does sucking

the fetus reaches half of its full length at birth

later fetus develops scalp hair replacing lanugo hair
infant period
The last period of growth, weeks 36 to 38.


fetus puts on body fat (white fat) insulating internal organs and stretching wrinkled skin into a smooth surface

brain size increases

lungs produce surfactant and are capable of sustaining the breathing of oxygen

ends with birth of infant

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