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| The following 3 periods reflect more precisely than "trimesters" the nature of fetal developmental and maternal physical changes observed in pregnancy: |
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| 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. Milestones: 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. Milestones: 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. Milestones: 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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