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, endocrine glands formed but not fully
functional,
myelination spinal roots and spinal cord begun, lungs gland like, 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