Fetal growth and parturition

Fetal growth

• Rate of growth in humans– Relatively slow during first 20 weeks– Rapidly increased during week 30-36– Slows down until birth– Different from domestic species

• Majority of growth takes place during the last trimester

• Accumulation of proteins– Muscle development– Precedes fat

deposition• Eventually exceeds

protein deposition• Energy storage

• Decreased placenta to fetus ratio

• Rate of growth– Fetal genome– Maternal factors

• Nutrients– Postnatal health– Postnatal development

• Placental lactogen– Stimulation of growth

Fetal metabolism

• Nutrients– Maternal source

• Glucose– Major energy source (50 % of total energy for fetal

growth)

• Amino acids• Fatty acids• Vitamins• Minerals

• Placenta– Physical barrier

• Diffusion of very small molecules and lipid-soluble molecules

• Large molecules and polar molecules – Transport system– Breach in the placenta

– Movement of molecules between maternal and fetal circulation

• Layers of cells between two circulations

Importance of fetal adrenal gland

Parturition

• Three stages– First stage

• Initiation of myometrium contraction• Initiated by the fetus

– Second stage• Expulsion of the fetus

– Third stage• Expulsion of fetal membrane

Parturition

• First stage– Initiated by fetus– Distress

• Maximum size that can be maintained– Lack of nutrients– Hypoxia

• Activation of fetal hypothalamus-pituitary-adrenal axis

– Release of ACTH by the fetal pituitary gland

• Fetal ACTH– Release of cortisol from

fetal adrenal gland• Fetal cortisol

– Removal of “progesterone block”

• Contraction of myometrium

• Conversion of progesterone to estradiol

– 17-hydroxylase– 17-20 lyase– Aromatase

• Fetal cortisol– Increase production of PGF2 by the

endometrium• Further removal of progesterone block

– Cause luteolysis

• Stimulates contraction of myometrium– Synergism with estradiol

• Contraction of myometrium– Pushes the fetus toward cervix

• Activation of the CNS– Secretion of oxytocin from the posterior pituitary gland

• Oxytocin– Increased contraction of the uterine smooth muscle

– Positive feedback• More contraction (pushing of fetus), increased oxytocin

secretion• Entering of the fetus in the cervical canal

– End of the first stage

Expulsion of fetus

• Relaxation of the cervix and pelvic ligaments– Relaxin

• Secretion stimulated by PGF2

• Role of estradiol on parturition– Increased overall secretory activity

• Mucus production by cervix and vagina– Removal of cervical plug– Lubrication

• Rupture of fetal membrane– Loss of amniotic and allantonic fluid

• Further lubrication

• Fetus becomes hypoxic– Movement of the fetus

• Stimulates further contraction of the myometrium– Increased strength of contraction

Expulsion of the fetal membrane

• Shortly after expulsion of the fetus– Dislodging of fetal membrane from the uterus– Vasoconstriction

Mammary gland development

• Cyclic changes in ovarian steroid hormones– Essential for mammary

development• Estradiol

– Development of mammary alveoli (secretory unit)

• Progesterone– Development of

mammary ducts

• Final stage of mammary gland development– Pregnancy

• Ovarian/placental steroid hormones

– Periparturient period• GH and prolactin

– Placental lactogen?

• Glucocorticoids

Lactation

• Parturition– Metabolic shift

• Fetal growth to synthesis and secretion of milk

– Mobilization of nutrients

• Storage within the body– Fats

• Triggered by changes in hormones

Lactation and re-initiation of reproductive cycle

• Lactating women– High prolactin– Low LH and estradiol

• No ovulation– Lactation-induced anovulation/amenorrhea (infertility)

– Women remain anovulatory as long as they breast-feed their babies

• Increased survivability of the infants

• Lactational anestrus in the domestic species– Divert nutrients for lactation

• Ensure the survival of offspring

– Lactation stimuli/physical contact between mother and offspring

– Animals remain in anestrus (absence of estrus) until nutrient intake exceeds nutrient demand