Physiology Changes in Pregnancy

• Most pregnancy related symptoms result from hormonally induced adaptations.
• Negative feedback system

  • maintains a constant level of activity
  • the majority of hormone control in the body
• Glucose regulation is a negative feedback system:

  • increased blood glucose
  • pancreas releases insulin
  • blood glucose falls
  • inhibits further insulin secretion
• Positive feedback systems are used when a temporary/intermittent boost in activity is needed
• Positive feedback systems:

  • Na+ influx in action potential
  • platelet activation in blood clotting
  • oestrogen secretion by follicle to trigger ovulation
  • oxytocin secretion in labour
  • prolactin secretion in breast-feeding
• GnRH travels from the hypothalamus to the anterior pituitary via the portal system. Initially this is low frequency pulsatile release however with positive feedback from oestrogen on the hypothalamus this becomes higher frequency.
• FSH release from the anterior pituitary is stimulated by GnRH from the hypothalamus. I
• FSH travels from the anterior pituitary to the ovaries in the bloodstream, where is stimulates growth and development of follicle.
• In days 12-14 of the menstrual cycle, estradiol released by a maturing follicle has a positive feedback effect on the hypothalamus and anterior pituitary, stimulating higher frequency release of GnRH, and therefore release of LH.
• LH release is stimulated by GnRH from the hypothalamus. A surge at day 14 of the menstrual cycle triggers ovulation and stimulates corpus luteum formation.
• In the luteal phase, the oocyte has been released and the corpus luteum releases progesterone to prepare the uterus for potential fertilisation.
• The corpus luteum is not sustained in the absence of fertilisation - menstruation occurs.
• Blastocyst produces beta-HCG which supports the corpus luteum. This continues to produce progesterone which sustains the uterus and makes it easier for the blastocyst to implant in the wall.
• beta-HCG and the corpus luteum are needed to sustain progesterone (and oestrogen) levels until placental production is sufficient at around 10-12 weeks.
• beta-HCG levels start to drop after about 3 months as it is no longer needed to maintain the corpus luteum.
• In the first three months after fertilisation, there is exponential increase in beta-HCG levels, as it is needed to maintain the corpus luteum.
• beta-HCG during pregnancy

  
• beta-HCG is a glycoprotein hormone rapidly secreted from trophoblast. It can be detected 10 days after fertilisation, so is used for pregnancy test. Levels of beta-HCG double every 2-3 days for in normal pregnancies.
• beta-HCG can be produced pathologically:

  • ectopic pregnancy
  • trophoblastic tumours
  • pineal tumours
• In ectopic pregnancies, beta-HCG levels do not drop after three months.
• Beta-HCG causes morning sickness.
• If progesterone fails for any reason, endometrium and developing embryo are shed, producing a miscarriage.
• After 10-12 weeks the placenta takes over from the corpus luteum in production of progesterone.
• The placenta produces:

  • oestrogen
  • progesterone
  • beta-HCG
  • human placental lactogen
• Oestrogen and progesterone are steroid hormones produced by the placenta.
• beta-HCG and human placental lactogen are peptide hormones produced by the placenta.
• Effects of pregnancy on the uterus:

  • smooth muscle relaxation
  • prevents fetal expulsion
  • cervical plug formation
  • microbial barrier
• Progesterone can relax smooth muscle in many parts of the body including the uterus, blood vessels, GI tract, ureters and bronchi.
• Progesterone causes the respiratory centre to become more active, driving hyperventilation in response to increased oxygen demand for both mother and fetus.
• Progesterone increases Na+ reabsorption in the kidneys via the renin-angiotensin system.
• Progesterone stimulates lobular tissue development in the breasts, but inhibits milk production.
• Progesterone is produced by the placenta from 10-12 weeks of pregnancy. Levels fall a few days before delivery.
• Oestrogen stimulates myometrial cell growth for growing fetus.
• Oestrogen is responsible for connexin insertion - electrochemical links (gap junctions) between myometrial cells in the uterus.
• Oestrogen is responsible for oxytocin receptor insertion in the uterus in preparation for labour.
• Oestrogen stimulates PGE2 production, which softens the cervix.
• Oxytocin stimulates breast duct development but inhibits milk production.
• Oestrogen synthesis during pregnancy is complex and involves more than just the placenta, because conversion of progestogen to androgen (DHEA) is not possible in the placenta.
• Synthesis of oestrogen

  • cholesterol
  • progestogen
  • androgen
  • oestrogen
• The adrenal cortex produces cortisol, which is stimulated by corticotropin (ACTH) from the anterior pituitary, which is stimulated by corticotropin releasing hormone (CRH) produced by the hypothalamus, and also the placenta during pregnancy.