Some landmarks in fetal development
1. Beginning of fetal period – 9th week
     The gut recanalizes, the midgut is herniated into umbilical cord, heart is partitioned,
     definitive aortic arches are formed, hematopoiesis ongoing in liver, diaphragm
     formed, ossification begun (clavicle), teeth are cap stage, tail has regressed, external
     genitalia indifferent, head is half crown rump length, there is a startle response
2. Beginning of the second trimester – 12 weeks
    The midgut no longer herniated, dilute urine by metanephros, liver produces serum
     albumin, stores glycogen, secretes bile,  is hematopoietic, endocrine glands formed
     but not fully functional, myelination spinal roots and spinal cord begun, lungs
     glandlike, fetus can hiccup, swallow, gasp, yawn, gender differences visible
3. Earliest time of possible survival – 26-28 weeks
    The gut has developed villi and crypts, peristalsis is present, surfactant secreted in
    lungs, alveolar walls thinning, hematopoiesis in liver, spleen and bone marrow,
    coordination of endocrines by pituitary, some myelination in brain, gyri, sulci are
    forming, fetus responds to sound, is covered by lanugo hair
4. End of fetal development - 38 weeks
     The pH of stomach becomes acid, some digestive enzymes present (e.g., lactase),
     meconium in bowel, hematopoiesis only in bone marrow, testes descended,
     myelination of corticospinal tracts to level of medulla, lanugo hair replaced with adult
     hair, vernix caseosa covers the body, fetus can suck, pupillary light reflex present