Parturition

Question 1

What is the role of placental progesterone?

Answer 1

Suppresses myometrial contractions throughout pregnancy

Promotes formation of the mucous plug in the cervical canal

Prepares mammary glands for lactation

Essential for gestation

Question 2

What is the role of placental oestrogen?

Answer 2

Proliferative effect on uterus and breasts

Prepares the uterus and cervix for labour

Question 3

Define labour

Answer 3

Process where foetus is expelled from the uterus

Question 4

How are placental oestrogen and progesterone different to the hormones produced by the corpus luteum?

Answer 4

Placental oestrogen and progesterone are produced by the placenta in an independent pathway.

Cholesterol from the maternal blood crosses the placenta, and some is converted by the placenta to progesterone which enters the maternal circulation.

Some cholesterol enters the foetal blood where it is converted to dehydroepiandrosterone-sulphate (DHEA-S) by the foetal andrenal cortex. This is converted to DHEA, then oestrogen by the placenta which enters the maternal circulation.

Question 5

When does the placenta take over production of oestrogen and progesterone?

Answer 5

Approximately week 8-12

Secretion of steroids by the corpus luteum falls after 8 weeks despite hCG stimulation.

Question 6

How does the uterus become activated?

Answer 6

Towards the end of pregnancy the synthesis of oestrogens from DHEAS accelerates.

The rise in oestrogens increases the number of oxytocin and prostaglandin receptors on the myometrium.

Oxytocin stimulates uterine contractions which are weak, irregular and painless - Braxton Hicks contractions

In labour, contractions are initiated by pacemaker cells.

Question 7

List the constituents of human milk

Answer 7

Water

Lactose

Fat

Lactalbumin

Lactoglobulin

Minerals (Ca2+, Fe, Mg, Na, K, P, S)

Vitamins

Question 8

Describe the mechanisms of milk production in the lactating breast

Answer 8

Cell proliferation of the breast increases during pregnangy under the influence of oestrogen, progesterone, prolactin, hPL, GH and insulin.

Lobues increase in number and size. After delivery prolactin activates transcrption of genes responsible for for milk proteins and lactose sythesis.

Luminal cells produce colostrum for the first 10 days which then becomes mature milk. Secretion of milk and maintenance of lactation depends on high concentrations of prolactin.

Question 9

Describe the hormonal control of the growth and development of mammary tissues

Answer 9

The mammary gland of the breast consists of many lobules with branched tubules that form alveoli. Several lobules drain into a single lactiferous duct that opens at the nipple.

The mammary glands begin to develop in puberty in response to ovarian oestrogen,which cause lactiferous ducts to proliferate into alveoli.

Most development of mammary tissue occurs in pregnancy. Oestrogen and progesteronepertrophy of the alveolar system which differentiate in mid-pregnancy.

Question 10

Control of milk production

Answer 10

Regulated by prolactin which stimulates transcription of genes for milk proteins and lactose synthesis.

Prolactin levels regulated by suckling which produces a neuro-endocrine reflex.

Question 11

Describe the mechanisms which produce cessation of lactation

Answer 11

Maintenance of lactation depends on regular suckling to promote prolactin secretion and remove accumulated milk

If suckling stops then milk production stops due to the drop in prolactin levels, and because the excess milk causes turgor-induced damage to the breast.

Milk secretion stops within a few days and the alveoli involute.

Question 12

Cervical ripening

Answer 12

Softening of the cervix promtoed by prostaglandins

Involved a reduction in collagen and increased glycosaminoglycans which decrease the aggregation of collagen fibres.

Collagen bundles loosen which allows the cervix to dilate.

Question 13

Initiation of labour

Answer 13

Towards the end of pregnancy the synthesis of oestrogens from DHEAS accelerates, which increases oestrogen levels.

Oestrogen increases the excitability of the myometrium which engages in weak, irregular, painless contractions and increase in frequency.

Prostaglandin synthesis by the myometrium also increases myometrial activity and promotes cervical ripening.

Question 14

Ferguson reflex

Answer 14

Initiated in by stretch receptors in the cervix in response to the pressure of the foetal head.

Stretching of the cervix triggers a large increase in oxytocin during the first stage of labour.This causes the smooth muscle of the uterus to contract, further stretching and dilating the cervix

Sensory receptors in the cervix and vagina are stimulated by contractions, excitation passes to the hypothalamus promoting further OT release.

This positive feedback makes contractions more foreceful and frequent.

Birth terminates the feedback.

Question 15

Hormones involved in the onset of labour

Answer 15

Prostaglandins - produced by the endometrium, enhance the release of calcium from intracellular stores

Oxytoxin: secreted from the posterior pituirary gland, lowers the threshold for triggering action potentials.
(low progesterone relative to oestrogen iincreases sensitivity of myometrium to oxytocin)

(synthesis increases with a rise in oestrogen or fall in progesterone)

Question 16

Describe the events in the first stage of labour

Answer 16

8-24 hours

The cervix becomes fully dilated (10cm) as the baby’s head is forced downwards by myometrial contraction

As teh cervix is stretched, a nervous reflex leads to oxytocin secretion from the posterior pituitary which further stimulates uterine contractions (positve feedback)

Contractions increase in force, frequency and duration as labour progresses

Question 17

Describe the events in the second stage of labour

Answer 17

30-90mins

Begins with full dilatation of the cervix and ends with birth of the baby.

Vaginal stretch stimulates reflex abdominal contractions. Can be reinforced voluntarily to co-incide with uterine contractions.

Question 18

Describe the events in the third stage of labour

Answer 18

15-30mins

Placenta shears off the wall of the contracted uterus

Myometrium contracts to limit blood loss

Question 19

How does milk composition change with time?

Answer 19

First week: colostrum - high protein and Igs, less water/fat/sugar (40ml per day)

Mature milk: Igs, proteins, fat, sugar, water, lactose, minerals, vitamins. Produced in response to demand

Question 20

Complications of preterm delivery

Answer 20

bronchopulmonary dysplasia: chronic lung disease (require supplemental oxygen >28days birth)

pneumothorax: collapsed lung

necrotising enterocolitis: necrosis of the intestine

Question 21

What hormones control lactation?

Answer 21

oxytocin: stimulates milk ejection
prolactin: stimulates milk secretion

Question 22

How is lactation prevented during pregnancy?

Answer 22

High levels of oestrogen and progesterone make the alveolar cells resistant to prolactin.

At birth, progesterone levels fall which makes the alveolar cells responsive to prolactin and milk is produced.

N.B. High levels of oestrogen and progesterone in early pregnancy stimular hypertrophy the ductal-lobar-alveolar system.

Question 23

How is lactation maintained?

Answer 23

Suckling triggers prolactin secretion by a neuroendocrine reflex pathway.

Suckling is detected by mechanoreceptors in the nipple which send positive signals to the hyptothalamus. This increases the release of PRH and decreases dopamine secretion, which increases prolactin.

Sucking also stimulates release of oxytocin from the posterior pituitary.

Results in increased milk production and milk ejection in response to demand.

In the absence of suckling, milk secretion stops within days and the alveoli involute.

Question 24

Milk ejection reflex

Answer 24

Stimulated by oxytocin

Suckling triggers release of oxytocin from posterior pituitary gland. OT stimulates contraction of myoepithelial cells around the alveoli, ejecting milk into the ductal system of the mammary glands.

Reflex can be conditioned by sight or sound of the baby.

N.B. catecholamines inhibit OT neurons, stress can prevent milk ejection even when the breasts are full.

Question 25

Describe the properties of uterine smooth muscle (myometrium) that facilitate labour

Answer 25

Myometrium is made up of bundles of smooth muscle. Action potentials spread from cell to cell via gap-junctions which allow coordinated contractions to spread over the myometrium.

Pacemaker cells in the mymetrium mean it is spontanously motile

During labour, at each contraction the muscle fibres shorten, but do not relax fully (brachystasis). This means the uterus (fundus) progressively shortens, which pushes the presenting part toward the birth canal

Question 26

Describe the immediate physiological chges that occur in the neonate which enable independent life

Answer 26

Within a minute of delivery the baby takes its first breath (triggered by trauma, cold, hypoxia)

This causes a dramatic fall in pulmonary vascular resistance, reducing the pulmonary arterial pressure and increasing left atrial pressure. This shuts to foramen ovale and rising pO2 causes the ductus arteriosus to close. The ductus venosus also constricts so all the blood entering the liver passes though the hepatic sinusoids.

Question 27

Describe the changes that occur to produce the birth canal

Answer 27

Sacroiliac ligaments and pubic symphysis soften so expansion of the pelvic cavity can occur.

Cervix thins and becomes dilated

Question 28

How is the condition of the neonate assessed?

Answer 28

Agpar score 0-10

asesses colour, tone, pulse, respiration, response