Summary (1) Twenty‐six patients whose labours have been judged to be normal have had samples of venous blood taken at the same time as samples of foetal scalp capillary blood at different stages of labour. (2) Using the Astrup micromethod, pH, base excess, and Pco2 estimations have been made on this blood. There is an initial improvement in the maternal and foetal values which is slowly reversed as labour progresses. (3) A constant maternal‐foetal acid‐base difference has been shown to exist at all stages of labour. (4) The suitability of foetal scalp capillary blood for these estimations is demonstrated by the similarity of the acid‐base values of foetal scalp blood taken at full cervical dilatation with the head in the pelvic cavity.

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Amniocentesis has enjoyed ever-increasing popularity since ,Bevis (1956) related the absorption of the spectrophotometric curve at 450 m/u with the prognosis of haemolytic disease of the newborn. In many countries estimation of liquor pigment levels has been preferred to maternal antibody titres as a guide for induction of labour (Walker, 1957;Cary, 1960;Freda, 1964); however, the general experience has been that the bile pigment level alone can be most misleading (Liley, 1963), particularly when specimens are contaminated with blood. A number of wrong predictions were made at Queen Charlotte's Hospital in 1965; these included five Rh-negative babies which had been predicted as affected Rh-positive. At that time the only information available on the " normal " pigment level in liquor amnii was from Rh-negative women with' isoantibodies other than Rh who gave birth to normal babies or from Rh-isoimmunized women who gave birth to Rh-negative babies (Walker and Jennison, 1962;Walker et al., 1964; Watson et al., 1965).Estimation was therefore. made of pigment levels in liquor amnii in a series of normal Rh-positive pregnant women. Patients and MethodsLiquor specimens from 29 Rh-positive normal pregnant women were examined during the 30th to 32nd weeks of gestation. These patients were all volunteers who were informed that the test was required purely for research purposes and was not part of their prenatal treatment. Specimens of liquor amnii were also taken from 22 Rh-negative mothers with histories of severe haemolytic disease and heterozygous husbands, or rising Rh-antiglobulin antibody titre, or with an antibody titre that conflicted with a history of previous offspring with haemolytic disease or of stillbirths. In less severe cases the specimens were taken during the 28th and 30th weeks of gestation, but in more severe cases likely to require intrauterine transfusion tests were made as early as the 20th week.About 10 ml. of liquor amnii was obtained by abdominal paracentesis, as described by Walker and Jennison (1962). The specimen was centrifuged, first in a clinical centrifuge to remove any red cells, and then the supernatant was recentrifuged at 17,000 r.p.m. (35,000 g) in a refrigerated (40 C.) centrifuge for 20 minutes to obtain a clear cell-free solution.The optical extinction (E) of the clarified liquor was measured against water as a blank in a 1-cm. cuvette at 454, 490, 520, and 574 my with a Unicam S.P.500 spectrophotometer. The total bile pigment concentration in the specimen was determined by diazo-coupling, as described by Watson (1962), except that 1.4 ml. instead of 1 ml. liquor was taken for the test and the reagents were increased proportionally. The total protein concentration was determined by the biuret reaction: 0.5 ml. of liquor (or 0.25 ml. where the protein concentration was found to be greater than 5 mg./ml.), diluted to 1 ml. with 0.9% w/v saline, was mixed with 1 ml. of biuret reagent, prepared by diluting stock reagent (Weichselbaum, 1946) one in five with 0.2 N sodium hydroxide co...

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