Biology of Reproduction

XXI. Labor and Parturition                          back to XX. Pregnancy

	A. Maturation of the fetal adrenal and adrenal axis

		1. fetal adrenal now produces mostly cortisol instead of DHEA

			a. CRH stimulates ACTH

			b. ACTH stimulates cortisol (F) from the fetal adrenal

		2. cortisol helps convert P to E in the placenta

			a. ­ adrenal cortisol (sheep) or androgens (humans) 

				i. F ® ­ P450C17b and 17-20 lyase: P ® E2

 				ii. adrenal androstenedione ® E2

			a. P levels fall at the end of pregnancy and E levels rise

		3. E/P ratio changes, becomes  > 1

		4. ­ E/P ratio causes:

			a. Oxy receptors to rise in the uterus

				i. labor begins

				ii. P inhibition of uterine contractions is now limited

			b. increased neural feedback from the cervix and uterus

				i. stimulates Oxy release

			c. increased production of Oxy

			d. increased production of PGF2a

			e. E treatment late in pregnancy speeds up onset of labor

		5. Cortisol levels in fetal plasma and amniotic fluid
		    rise late in pregnancy

			a. Delayed newborns commonly have underdeveloped 
			    adrenals, lower than normal cortisol,
			    and the mothers have low E levels

			b. Premature newborns tend to have enlarged adrenals

			c. maternal stress (chronic ® ­ F) may
			    stimulate premature labor

			d. injection of ACTH or cortisol will stimulate
			   labor in other animals (e.g. sheep)

	B. Oxytocin

		1. Labor begins with an increase in Oxy-R not in Oxy

			a. ­ Oxy-R stimulated by ­ E/P ratio

				i. directly by cortisol?

		2. Oxy stimulates Uterine contractions

			a. Oxy (or Pitocin) will induce labor

				i. Ethyl Alchol or other Oxy inhibitors will delay labor

			b. acute stress ® ­ Epi and NE block uterine contractions

		3. Uterine contractions = Fetal Ejection Reflex stimulate Oxy release

			a. also stimulated by ­ E/P ratio

		4. Oxy stimulates uterine PGF2a production

	C. Prostaglandins

		1. PGF2a stimulates uterine contractions

			a. PGF2a or PGE2 administration will initiate labor

			b. aspirin or other anti-prostaglandins delay labor

		2. burst of PGF2a  with each contraction during labor

			a. contractions stimulate PGF2a   secretion

		3. ­ E/P ration stimulates uterine PGF2a production

	D. Fetal Ejection Reflex

		1. stimulation or distention of cervix, uterus, or vagina
		    results in neural feedback turning on the secretion of Oxy

		2. stimulating b2-adrenergic receptors in myometrium
		    blocks contraction

			a. Epi and NE from adrenal medulla during acute stress

				i. lamaze and Bradley methods utilized this idea

					(1) focusing on breathing style and rhythm reduces nervousness

						(a) thereby ¯ Epi and NE

XXII. Lactation