26.     Physiological changes in pregnancy.

See also MCQ1, question 43 "Iron & Pregnancy". 

Total blood volume increases by about 30% in normal pregnancy.

The increase is mainly in plasma volume ~ 50%.

The red cell mass rises, but only by about 20%.

This leads to the well-known haemodilution of pregnancy.

A bit like the publican watering the beer.

It also leads to differing views of what constitutes anaemia in pregnancy.

Nelson-Piercy in the Handbook of Obstetric Medicine says that levels < 10.5 gm. / dl. should be regarded as abnormal.

Multiple pregnancy exaggerates these changes.

Blood pressure does not rise significantly.

Indeed, it tends to fall in the second trimester.

So there must be a substantial fall in peripheral resistance to accommodate the increased cardiac output.

Uterine blood flow increases hugely.

And the uterine vasculature is low-resistance if trophoblastic invasion of the spiral arteries has occurred properly.

See MCQ2, question 3 "Pre-eclampsia" and MCQ10, question 1 "Trophoblasic invasion of the spiral arteries".

There is also the 50% increase in renal blood flow to reduce peripheral resistance.

The pulse rate rises by about ten beats per minute.

Renal blood flow increases by up to 50% and is reflected in lower urea and creatinine levels in pregnancy.

The white cell count rises during pregnancy and further in the puerperium.

Nelson-Piercy (page 311) gives figures of:

    4 – 11 for the non-pregnant,

    and 6 – 16 in pregnancy.

James (page 1680) gives figures of:

    4 – 9 for the non-pregnant,

    9 for pregnancy,

    and 16 for the puerperium.

The relevant units are 10⁹ /litre.