OB-Physiological Changes
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þ Cardiovascular changes of pregnancy
 - Intravascular volume increases by 50%.
 - And, red cell mass increases by 25%
 - Above leading to hematocrit decreasing by about 5%.
 - Heart rate increases to 80-95
 - SBP drops 0-15 mmg, DBP drops 10-20, therefore pulse pressure increases
 - 1/6 or 2/6 systolic murmur is normal.
 - EKG:  left axis deviation, flat T waves in II, Q waves in III and F.
 - symptoms of shock may be delayed due to pre-existing vascular changes.
 - famous "supine hypotensive syndrome": supine position decreases venous
   return due to pressure on inferior vena cava, so minimize time spent on the
   back.

þ Pulmonary changes of pregnancy
 - Diaphragm elevated about 4 cm in 3rd trimester
 - Increase in AP diameter of chest
 - pCO2 decreases to about 33; HCO3-- decreases to 19 in compensation
 - pO2 increased by 10 mm Hg
 - increased 2,3 DPG levels to permit better O2 delivery to fetus.
 - progesterone relaxes the esophageal sphincter so INCREASED RISK OF
   ASPIRATION during intubation

þ Renal changes of pregnancy
 - physiological hydronephrosis from uterine pressure and progesterone
   effects.

þ Abdominal changes of pregnancy
 - Decreased sensitivity of the peritoneum to irritation may mask signs of
   early peritonitis.

þ Pregnancy Dates
 - 40 weeks from last period (38 from conception)
 - pregnancy tests usually turn positive about the time of the first missed
   period.
 - Quantitative HCG doubles about every 36-48 hours for the first 9 weeks.
 - Plateau of HCG in first nine weeks suggests ectopic or fetal demise.
 - Fetal heart tones first audible by Doppler at 12 weeks.
 - Uterus first palpable above pubis at 12 weeks.
 - Uterus reaches umbilicus at 20 weeks.
 - After 20 weeks distance of uterus above pubis in cm = weeks of gestation

þ Neonatal survivability (Magee, 1991)

Gestational age, weeks          Neonatal Survival, weeks
==========================================================================
        20                              0%
        22                              7%
        24                              54%
        26                              88%
        28                              91%
        30                              96%
        32                              99%
        >32                             99%