Physiology Of Pregnancy

Physiology of Pregnancy EDD = + 1yr & 10 days from LMP – 3 months Respiratory System ▲O2 consumption by 15-20%, achieved by hyperventilation (minute vol ▲by 40% by ▲tidal vol) CO2 ▼ (favours transfer of CO2 from fetus to mother) Renal excretion of HCO3- and Na+ increases to maintain pH Dyspnoea common (uterus restricts diaphragm movements) NB: Beware PE Cardiovascular ▲CO by 40% (to 6l/min) by ▲ in stroke vol & HR by start of 2nd trimester. High flow à higher O2 to placenta for fetus. ▲ flow to skin à heat loss to compensate for ▲ metabolic rate, skin feels warm ▼peripheral vascular resistance due to structure of placenta Together à ▼BP in 2nd trimester (can rise again in 3rd trimester) NB: Supine hypotension due to uterus occluding inf vena cava Blood 25% ▲ total red cell mass, 40% ▲ plasma vol à dilutional drop in Hb conc & Haematocrit ▼ in plasma colloid osmotic pressure à oedema ▲ Iron requirements, ▼ serum ferritin à give iron supplements ▲ Folate requirements (deficiency may occur) à give supplements to prevent NTD Haemostasis Hypercoagulable state, increased risk TED ▲ fibrinogen, factor VIII, von Willebrand factor, ▼ antithrombin III ▲ fibrinolysis à ▲ turnover of coagulation factors Slight ▼ in platelets but ▲ aggreability Renal ▲ renal blood flow and GFR by 60% from early 1st trimester à ▼ plasma creatinine & urea ▲ in aldosterone levels prevents loss of Na+ ▲ glycosuria because filtered load of glucose exceeds re-absorption capacity Dilatation of renal pelvis & ureters due to progesterone + pressure à urine stasis à ▲UTI Gastrointestinal Nausea & Vomiting common in early pregnancy, Pica = cravings Delayed gastric emptying (NB: ▲ risk of aspiration if GA req) ▼ gastric acid à PUD improves ▼ lower oesophageal sphincter pressure & ▲ intra-abdo pressure à ▲ severity reflux ▼ gut motility (progesterone = relaxant à?constipation) & ▲ transit time à ▲ absorption ▲ straining, ▲ intra-abdo pressure & vasodilatation à rectal haemorrhoids Gum changes à free NHS dental during pregnancy Hepatic Pregnancy is a cholestatic state ▲ Alk Phos (almost double) mostly due to placental secretion ▲ serum cholesterol (due to oestrogen) à bile salt production + ▼ motility of GB (progesterone) à ▲ risk of galls stones Skin & Appendages ▲ blood flow to lose extra heat generated by ▲ metabolism ▲ pigmentation in nipples & linea nigra Striae gravidarum (initially red, fade to silvery after delivery) ▲ proportion of growing hair à “coming out in handfuls” postnatally Metabolic Changes ▲ metabolic rate (20%) to support fetus & ▲ maternal requirements, weight ▲ approx 12kg Initial ▲ insulin sensitivity à glycogen synthesis & fat deposition Later insulin resistance à ▲ fetal glucose, ▲ maternal serum lipids (alternative energy) Pregnancy is diabetogenic & potential for gestational diabetes (cortisol, progesterone, oestrogen & HPL are all insulin antagonists à ▲ glucose level) ▲ iodine uptake & ▲ serum T3 & T4, but oestrogen à ▲ thyroid binding globulin free serum T3 & T4 stay in normal range or fall Ca2+ transfer across placenta is active process (against conc gradient). Ca2+ absorption from diet increases to meet need. Due to ▲ blood vol maternal Ca2+ conc decreases Placental Function Fetal Hb has a higher O2 affinity than maternal à higher saturation at given O2 level Fetal Hb conc is higher than maternal favours O2 transfer to fetus CO2 fetal à maternal increases maternal dissociation (bohr effect) Low maternal CO2 levels (see earlier) favours CO2 fetal à maternal Glucose transfer by facilitated diffusion, AA, FFA & vitamins transferred by active transport HCG maintains corpus luteum, Oestrogen à uterine growth, cervical changes and breast development, Progesterone à smooth muscle relaxation (uterus, GI, ureters), HPL à mobilises FFA to improve gluose supply for fetus Weight gain Approx 12.5kg, mostly in last 20 weeks Fat 3.5kg Fetus 3.3kg ECF 2.6kg Blood 1.2kg Uterus 900g Placenta 600g Breasts 400g