Physiological Aspects of Normal Pregnancy

Definitions for dating pregnancy

Conception: 2 weeks after 1st day of last menstrual period (LMP) with a regular 28 day cycle

Estimated due date (EDD): Naegele’s rule – add 9 months + 7 days to 1st day of LMP. Pregnancy lasts 38wks from conception/40wks from 1st day of LMP


  • 1st trimester = 1-12wks
  • 2nd trimester = 13-27wks
  • 3rd trimester = 28-40wks



  • Delivery –> 6wks.
  • Reversal of the physiological changes of pregnancy

Maternal physiological changes during pregnancy

Cardiovascular and respiratory changes

  • 40% increase in plasma volume by 32wks. [RBC] increases by 20%.
  • 40% rise in cardiac output. CO and BP fall if supine due to vena cava compression.
  • Reduced peripheral venous return causes BP drop in early pregnancy. Return to pre-pregnancy level later.
  • 40% increase in tidal volume.
  • Haemodilution: overall amount of Hb rises, but concentration falls.
  • O2 demand increases by 15%.
  • Increased clotting risk: increased factor VII, VIII, X, and rise in fibrinogen.
  • Increased RBC mass: protects against the ~0.5L delivery blood loss (1L if twins or CS)

Renal changes

  • Renal pelvis + ureters dilate (pressure/progesterone): risk of acute pylonephritis.
  • GFR increases by 50%: reduces plasma urea, creatinine and osmolality.
  • Increased urinary protein loss. >500mg in 24hrs is abnormal however.

Endocrine changes

  • Insulin secretion doubles. Physiological glycosuria may occur.
  • Thyroid binding globulin doubles. T3 + T4 fall slightly. Goitre more common.
  • Anterior pituitary doubles in size – risk of ischaemia in PPH (Sheehans syndrome)
  • Rise in total and free serum cortisol and urinary free cortisol

Musculoskeletal and skin changes

  • Joints of the lower back and pelvis soften.
  • Increased incidence of rashes/epistaxis/hyperpigmentation/spider naevi/erythema

Calcium and phosphate

  •  Increased demand of Ca (especially in 3rd trimester and puerperium) leads to increased gut absorption. Calcium is actively transported across placenta.
  • Serum Ca + phosphate levels fall (bound to albumin). Ionised Ca remains stable


  • Hepatic blood flow unchanged
  • ALP levels rise by 50% and albumin falls by 10g/L (causes a fall in total protein)

Uterine physiology

  • Morula becomes blastocyst at the 32 cell stage
  • Implantation 7-14 days post conception: blastocyst attaches and trophoblast cells invade the endometrium.
  • Organogenesis: 2-8 wks post conception.
  • Inner cell mass becomes embryo. Trophectoderm becomes placental trophoblast.
  • Foetus develops in amniotic cavity, attached to placenta by umbilical cord.
  • Amnion: membrane lining of cavity, expands as placenta progresses. 2nd layer (chorion) in apposition to the amnion.
  • Placenta is anchored to maternal decidua.
  • Intervillous space supplied by maternal spiral arteries.
  • Cord has 2 arteries (deoxygenated blood from fetus to placenta) + 1 vein (oxygenated blood from placenta to foetus).
  • Uterus holds 5L at term (500x pre-pregnancy): muscle hypertrophy.
  • Blood supply from uterine + ovarian arteries.
  • Cervical mucous plug protects during pregnancy.

Multiple pregnancy

  • 10X higher risk of stillbirth
  • 50% pre-term
  • Increased risk of IUGR

Dizygotic = non-identical, duplication of normal processes, dichorionic, diamniotic.

Monozygotic = earlier split, more independent. DCDA <3 days, 4-7days MCDA, >8d MCMA. Requires tertiary centre care if monozygotic!

Risks to mother:

  • Anaemia
  • Pre-eclampsia
  • Hyperemesis
  • Polyhydramnios
  • Complicated labour


Risks to developing foetuses:

  • Congenital abnormalities
  • Twin-to-twin transfusion
  • Pre-term delivery
  • Twin entrapment

More regular antenatal checks are required.

Foetal growth

Growth governed by intrinsic (maternal height/weight/ethnicity, fetal sex/genes/conditions) and extrinsic (environmental – social class, nutrition, maternal disease) factors.

Small for Gestational Age – a foetus that has failed to achieve a specific biometric or estimated weight threshold by a specific gestational age – may be constitutional or due to intra-uterine growth restriction (IUGR).

Assessments of foetal growth: Biparietal distance, head circumference, abdominal circumference, femur length – serial measurements more useful to gauge velocity. Plotted on centiles – can show if any dropping off/lag/acceleration.

Assessment of baby’s coping: kick charts, CTG, biophysical profile (fetal breathing movements, fetal movements, fetal tone, amniotic fluid volume).

Assessment of baby’s nutrition: placental assessment – appearance, blood flow characteristics via Doppler (umbilical – fetus to placenta, uterine – mother to placenta). Progressively greater resistance leads to absent or reversed End Diastolic Flow.  (read up on this if you’re interested, I won’t go into it further!)

Foetal development

Early pregnancy

  • Day 14 = ovulation
  • Fertilisation occurs commonly in the fallopian tube
  • Cell division occurs: zygote → morula → blastocyst as moving to uterine cavity
  • Day 23 = implantation – beginning of fetal-maternal dialogue
  • When the blastocyst implants – production of hCG by the decidua stimulates the ovary to produce progesterone (causes modification of maternal physiology).
  • hCG levels rapidly rise <10wks. Can be detected in serum/urine 4 weeks after LMP (urine PT +ve when concentration of hCG is 25IU/ml)


  • 4-5 weeks – gestation sac ~6mm
  • 5-6 weeks – yolk sac
  • 6 weeks – foetal pole ~5mm
  • 7 weeks – foetal heart activity
  • 8 weeks – limb buds, fetal movements
  • Foetus should double in size every week until 12 weeks gestation


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