Physiology of Pregnant Mother - Trimesters

Question 1

How long is the period of gestation?

List some physical changes within the mother to accommodate the demands of pregnancy:

Answer 1

Gestation - 38 weeks form conception, 40 weeks from last menstrual period

• uterine enlargement
• breasts enlarge and develop ability to produce milk
• volume of blood increases by 30%
• weight gain (mainly due to uterus and blood increase)
• respiratory activity increases ~20%
• urinary output increases and the kidneys secrete additional wastes from the fetus
• increased nutritional requirements - 5, 10 and 25% more in 1st, 2nd and 3rd trimester respectively

Question 2

What does the process of parturition require?

Answer 2

1. Dilation of the cervical canal to accommodate passage of the fetus from the uterus through the vagina to the outside
2. Contractions of the uterine myometrium that are strong enough to expel the fetus

Question 3

What are the changes during late gestation in preparation for parturition?

Answer 3

Braxton Hicks contractions: occur during last trimester

• mild contractions experienced with increasing strength and frequency, sometimes regular enough to be mistaken for onset of labour

Softening of the cervix:

• dissociation of cervical collagen fibres allows cervix to dilate during labour
• caused largely by relaxin (hormone secreted by CL) and by placenta
• relaxin also relaxes the birth canal by loosening cinnective tissue between pelvic bones

Fetus shifts downwards so top of head engages with cervix

• in a breech birth, any part of the body other than the head approaches the birth canal first

Question 4

What is the first stage of labour?

Answer 4

First stage: cervical dilation

• membrane surrounding amniotic sac ruptures, amniotic fluid escapes out of the vagina and helps lubricate the birth canal
• cervix is forced to dilate to accommodate the diameter of the baby’s head (max~10cm) - may take many hours

Question 5

What is the second stage of labour?

Answer 5

Second stage: Delivery

• begins once cervical dilation is complete, usually takes 30-90 mins
• baby moves through the cervix to the vagina
• stretch receptors in the vagina activate a neural reflex that triggers contraction of the abdominal wall in synchrony with the uterine contractions
• mother can also voluntarily contract abs

After birth, the baby is freed from the placenta by cutting the umbilical cord. Cord is tied and forms the umbilicus (navel)

Question 6

What is the third stage of labour?

Answer 6

Third stage: Delivery of the placenta

• shortly after delivery, a second series of uterine contractions separates the placenta from the myometrium and expels it through the vagina (afterbirth), completed within 15-30 mins
• after placenta is expelled, continued contractions of the myometrium constrict the iterine blood vessels at site of placental attachment to prevent haemorrhage

Question 7

What is involution?

What is it induced by?

Answer 7

• after delivery, uterus shrinks back to its pre-gestational size
• takes 4-6 weeks

Induced by:

• fall in oestrogen and progesterone when the placenta is lost at delivery
• facilitated in mothers who breast feed, oxytocin promotes myometrial contractions that help maintain uterine muscle tone, enhancing involution

Question 8

What triggers development of the breast?

Answer 8

During pregnancy:

• oestrogen promotes extensive duct development
• progesterone stimulates abundant lveolar-lobular formation
• prolactin (ant. pituitary hormone) and human chorionic somatomammotropin (hCS placental hormone) induce the sunthesis of enzymes needed for milk production
• prolactin also stimulates milk production after parturition

Question 9

How does suckling affect hormones?

Answer 9

Suckling triggers a neuroendocrine reflex that leads to secretion of:

• prolactin (stimulates milk production)
• oxytocin (stimulates milk ejection): stimulates contraction of myoepithelial cells, hastens involution, suppresses LH and FSH secretion - suppresses menstrual cycle

Question 10

What are the advantages of breastfeeding for the mother?

Answer 10

• oxytocin release triggered by nursing hastens uterine involution
• prolactin inhibits GnRH, thereby suppressing FSH and LH secretion
• lactation tends to prevent ovulation, decreasing the liklihood of another pregnancy (although not reliable method of contraception)