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