Transplacental haemorrhage

Performance of the Kleihauer Betke test in the prediction of neonatal anemia

OBJECTIVES

The aim of this study was to correlate antenatal Kleihauer (KT) test results with fetal hemoglobin at birth to find a threshold for predicting severe fetal anemia. The secondary objectives were to assess the impact of KT on obstetric management and to study the correlation between the middle cerebral artery peack systolic velocity and fetal anemia.

RESULTS

One thousand forty-six KT were positive over the 10-year period, but only 147 were included from 88 patients, of which 17 fetuses were anemic. Demographic and obstetric characteristics were similar between anemic and non-anemic groups. As regards new-born, there was a higher risk of prematurity among anemic as long as a lower birth rate in accordance. While a negative correlation was observed between KT and hemoglobin at birth, no KT upper threshold could be found that was both sensitive and specific. In addition, there was no case of fetal anemia when KT was repeated, even though it increased. KT showed little usefulness in obstetrics management to help improving neonatal care for anemia. Conversely, the MCA PSV demonstrated good performance in this matter and the ROC curve area was 0.91 (figure).

DISCUSSION

Feto-maternal hemorrhage is a rare but grave pathology which could lead to anemia. The most common clinical sign is reduced fetal movement and it was the main indication to perform a KT. Cardiotocography patterns suggestive of anemia are sinusoidal, micro-oscillatory and non-reactive monitoring. Ultrasound features were polyhydramnios, hydrop fetalis and increased MCA peack systolic velocity. KT was correlated with MCA PSV and with hemoglobin level at birth. However, the latter showed a better diagnostic performance. MCA PSV measurement is a powerful test to screen for fetal anemia, and should be part of the regular training of obstetricians. Indeed, this technic gives immediate and reliable results, while those of KT are delayed.

CONCLUSION

The KT should not be used as a tool to screen for fetal anemia but rather as a test to explain a fetal anemia. However, the MCA PSV is reliable in this matter and give immediate result, thus obstetrician should be trained to routinely perform it.

Anti-D monoclonal antibodies from 23 human and rodent cell lines display diverse IgG Fc-glycosylation profiles that determine their clinical efficacy

Anti-D immunoglobulin (Anti-D Ig) prophylaxis prevents haemolytic disease of the fetus and newborn. Monoclonal IgG anti-Ds (mAb-Ds) would enable unlimited supplies but have differed in efficacy in FcγRIIIa-mediated ADCC assays and clinical trials. Structural variations of the oligosaccharide chains of mAb-Ds are hypothesised to be responsible. Quantitative data on 12 Fc-glycosylation features of 23 mAb-Ds (12 clones, 5 produced from multiple cell lines) and one blood donor-derived anti-D Ig were obtained by HPLC and mass spectrometry using 3 methods. Glycosylation of mAb-Ds from human B-lymphoblastoid cell lines (B) was similar to anti-D Ig although fucosylation varied, affecting ADCC activity. In vivo, two B mAb-Ds with 77–81% fucosylation cleared red cells and prevented D-immunisation but less effectively than anti-D Ig. High fucosylation (>89%) of mouse-human heterohybridoma (HH) and Chinese hamster ovary (CHO) mAb-Ds blocked ADCC and clearance. Rat YB2/0 mAb-Ds with <50% fucosylation mediated more efficient ADCC and clearance than anti-D Ig. Galactosylation of B mAb-Ds was 57–83% but 15–58% for rodent mAb-Ds. HH mAb-Ds had non-human sugars. These data reveal high galactosylation like anti-D Ig (>60%) together with lower fucosylation (<60%) as safe features of mAb-Ds for mediating rapid red cell clearance at low doses, to enable effective, inexpensive prophylaxis.

Human platelet antigen (HPA)-1a peptides do not reliably suppress anti-HPA-1a responses using a humanized severe combined immunodeficiency (SCID) mouse model

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) occurs most frequently when human platelet antigen (HPA)-1a-positive fetal platelets are destroyed by maternal HPA-1a immunoglobulin (Ig)G antibodies. Pregnancies at risk are treated by administration of high-dose intravenous Ig (IVIG) to women, but this is expensive and often not well tolerated. Peptide immunotherapy may be effective for ameliorating some allergic and autoimmune diseases. The HPA-1a/1b polymorphism is Leu/Pro33 on β3 integrin (CD61), and the anti-HPA-1a response is restricted to HPA-1b1b and HLA-DRB3*0101-positive pregnant women with an HPA-1a-positive fetus. We investigated whether or not HPA-1a antigen-specific peptides that formed the T cell epitope could reduce IgG anti-HPA-1a responses, using a mouse model we had developed previously. Peripheral blood mononuclear cells (PBMC) in blood donations from HPA-1a-immunized women were injected intraperitoneally (i.p.) into severe combined immunodeficient (SCID) mice with peptides and HPA-1a-positive platelets. Human anti-HPA-1a in murine plasma was quantitated at intervals up to 15 weeks. HPA-1a-specific T cells in PBMC were identified by proliferation assays. Using PBMC of three donors who had little T cell reactivity to HPA-1a peptides in vitro, stimulation of anti-HPA-1a responses by these peptides occurred in vivo. However, with a second donation from one of these women which, uniquely, had high HPA-1a-specific T cell proliferation in vitro, marked suppression of the anti-HPA-1a response by HPA-1a peptides occurred in vivo. HPA-1a peptide immunotherapy in this model depended upon reactivation of HPA-1a T cell responses in the donor. For FNAIT, we suggest that administration of antigen-specific peptides to pregnant women might cause either enhancement or reduction of pathogenic antibodies.

Accurate quantitation of D+ fetomaternal hemorrhage by flow cytometry using a novel reagent to eliminate granulocytes from analysis

BACKGROUND

Quantitation of fetomaternal hemorrhage (FMH) is performed to determine the dose of prophylactic anti-D (RhIG) required to prevent D immunization of D- women. Flow cytometry (FC) is the most accurate method. However, maternal white blood cells (WBCs) can give high background by binding anti-D nonspecifically, compromising accuracy.

STUDY DESIGN AND METHODS

Maternal blood samples (69) were sent for FC quantitation of FMH after positive Kleihauer-Betke test (KBT) analysis and RhIG administration. Reagents used were BRAD-3-fluorescein isothiocyanate (FITC; anti-D), AEVZ5.3-FITC (anti-varicella zoster [anti-VZ], negative control), anti-fetal hemoglobin (HbF)-FITC, blended two-color reagents, BRAD-3-FITC/anti-CD45-phycoerythrin (PE; anti-D/L), and BRAD-3-FITC/anti-CD66b-PE (anti-D/G). PE-positive WBCs were eliminated from analysis by gating. Full blood counts were performed on maternal samples and female donors.

RESULTS

Elevated numbers of neutrophils were present in 80% of patients. Red blood cell (RBC) indices varied widely in maternal blood. D+ FMH values obtained with anti-D/L, anti-D/G, and anti-HbF-FITC were very similar (r = 0.99, p < 0.001). Correlation between KBT and anti-HbF-FITC FMH results was low (r = 0.716). Inaccurate FMH quantitation using the current method (anti-D minus anti-VZ) occurred with 71% samples having less than 15 mL of D+ FMH (RBCs) and insufficient RhIG calculated for 9%. Using two-color reagents and anti-HbF-FITC, approximately 30% patients had elevated F cells, 26% had no fetal cells, 6% had D- FMH, 26% had 4 to 15 mL of D+ FMH, and 12% patients had more than 15 mL of D+ FMH (RBCs) requiring more than 300 μg of RhIG.

CONCLUSION

Without accurate quantitation of D+ FMH by FC, some women would receive inappropriate or inadequate anti-D prophylaxis. The latter may be at risk of immunization leading to hemolytic disease of the newborn.

The maternal immune response to fetal platelet GPIbα causes frequent miscarriage in mice that can be prevented by intravenous IgG and anti-FcRn therapies

Fetal and neonatal immune thrombocytopenia (FNIT) is a severe bleeding disorder caused by maternal antibody-mediated destruction of fetal/neonatal platelets. It is the most common cause of severe thrombocytopenia in neonates, but the frequency of FNIT-related miscarriage is unknown, and the mechanism(s) underlying fetal mortality have not been explored. Furthermore, although platelet αIIbβ3 integrin and GPIbα are the major antibody targets in immune thrombocytopenia, the reported incidence of anti-GPIbα-mediated FNIT is rare. Here, we developed mouse models of FNIT mediated by antibodies specific for GPIbα and β3 integrin and compared their pathogenesis. We found, unexpectedly, that miscarriage occurred in the majority of pregnancies in our model of anti-GPIbα-mediated FNIT, which was far more frequent than in anti-β3-mediated FNIT. Dams with anti-GPIbα antibodies exhibited extensive fibrin deposition and apoptosis/necrosis in their placentas, which severely impaired placental function. Furthermore, anti-GPIbα (but not anti-β3) antiserum activated platelets and enhanced fibrin formation in vitro and thrombus formation in vivo. Importantly, treatment with either intravenous IgG or a monoclonal antibody specific for the neonatal Fc receptor efficiently prevented anti-GPIbα-mediated FNIT. Thus, the maternal immune response to fetal GPIbα causes what we believe to be a previously unidentified, nonclassical FNIT (i.e., spontaneous miscarriage but not neonatal bleeding) in mice. These results suggest that a similar pathology may have masked the severity and frequency of human anti-GPIbα-mediated FNIT, but also point to possible therapeutic interventions.

Placental immunology and maternal alloimmune responses

During pregnancy, women are tolerant of their semi-allogeneic fetus whilst not being immunosuppressed and indeed readily form alloantibodies. This 'Immunological Paradox of Pregnancy' may be explained by an understanding of placental anatomy and immunology. Trophoblast cells form the interface between the fetus and maternal tissues and blood and escape allorecognition because they lack classical human leucocyte antigen (HLA) class I and II molecules. Local immunoregulation, or tolerance, in the decidua is mediated partly by HLA-G(+) extravillous trophoblasts (EVT) that invade the tissue and prevent killing by maternal natural killer cells, cytotoxic T cells and macrophages. Placental hormones orchestrate the composition and regulatory function of maternal immune cells. In contrast, syncytiotrophoblast cells at the surface of chorionic villi, in contact with maternal blood, maintain a state of mild maternal systemic immunity via activation of innate immunity and skewing towards humoral immunity. This enables maintenance of a healthy immune system in pregnant women and robust protective antibody responses to pathogens whilst enabling survival of the fetus. However, this has the unfortunate consequence that pregnant women readily form alloantibodies to incompatible alloantigens on fetal red cells, platelets and leucocytes if fetomaternal haemorrhage (FMH) occurs. The antibodies are initially low affinity but after re-immunization with further FMH become functionally effective, high-titre IgG.

Ultrastructural localization of glycoprotein IIIa (GPIIIa, beta 3 integrin) on placental syncytiotrophoblast microvilli: implications for platelet alloimmunization during pregnancy

BACKGROUND

Fetal and neonatal alloimmune thrombocytopenia due to anti-human platelet antigen (HPA)-1a more commonly occurs in first pregnancies, unlike hemolytic disease of the newborn. Anti-D is produced after D+ fetomaternal hemorrhage; this usually occurs at parturition. Anti-HPA-1a could develop during pregnancy if maternal immunization is stimulated by HPA-1a expressed not only on platelets but also on other fetal cells.

STUDY DESIGN AND METHODS

An ultrastructural study of fetal placental chorionic villi was undertaken to determine the localization of glycoprotein (GP)IIIa carrying the HPA-1a/1b polymorphism. First trimester and term villi were incubated with a monoclonal antibody (MoAb) to GPIIIa or with positive control MoAbs (anti-placental alkaline phosphatase and ED822 MoAb) to villous syncytiotrophoblast (ST). Binding of MoAbs was detected with a gold-conjugated secondary antibody before processing the tissues and examination of ultrathin sections in an electron microscope.

RESULTS

Gold particles were evident on microvilli on the apical surface of ST when labeled with anti-GPIIIa and the placenta-specific MoAbs but not with an isotype control antibody. Immunolabeling for anti-GPIIIa on first trimester ST was similar to that of term ST.

CONCLUSION

The apical surface of the ST is bathed in maternal blood. During the natural regenerative process of human placenta, senescent parts of the ST are shed into maternal blood during pregnancy. This includes both apoptotic ST nuclei and microparticulate ST debris. The presence of GPIIIa on this circulating ST cellular material could be the source of HPA-1a alloantigen causing primary immunization of susceptible primigravidae early enough for anti-HPA-1a to cause fetal thrombocytopenia during a first pregnancy.

Antenatal administration of Rh-immune globulin causes significant increases in the immunomodulatory cytokines transforming growth factor-β and prostaglandin E2

BACKGROUND

Production of specific cytokines in response to administration of Rh-immune globulin (RhIG) was examined to assess the mechanism of inhibition of the anti-D production and prevention of hemolytic disease of the newborn (HDN).

STUDY DESIGN AND METHODS

Plasma levels of 17 different cytokines before and 48 hours after antenatal administration of anti-D were measured in 10 women candidates for prophylaxis with RhIG.

RESULTS

No striking changes were observed in levels of the cytokines interleukin (IL)-1 sRII, IL-12 p40, IL-16, or monocyte chemoattractant protein-1. Levels of IL-4, -5, -10, -13, and -17; macrophage inflammatory protein-1alpha; granulocyte-macrophage-colony-stimulating factor; tumor necrosis factor-beta; and interferon-gamma remained below detection levels both before and after testing. IL-1ra levels, however, showed a slight to moderate decrease in 7 of 10 women after RhIG administration. In contrast, levels of TGF-beta1 increased more than 1.3-fold in 7 of 10 women and more than 2-fold in 4 of 10 women; in 1 instance the increase was more than 5-fold and this woman also had a significant increase in TGF-beta2. In addition to TGF-beta, 5 of 10 women had a modest increase (>1.5-fold) in prostaglandin E2 (PGE2). Analyses of the combined results of the 10 women showed that increases in both TGF-beta1 and PGE2 after RhIG were significant.

CONCLUSION

These results indicate that RhIG prophylaxis can induce higher than baseline levels of two strongly immunomodulatory cytokines, TGF-beta and PGE2. These findings represent one possible mechanism for the inhibition of the primary immune response to the D antigen in women receiving RhIG prophylaxis for prevention of HDN.

Audit of anti-D immunoglobulin administration to pregnant Rhesus D negative women following sensitising events

We performed a retrospective audit of 99 Rhesus D (RhD) negative women to discover if anti-D immunoglobulin (anti-D) prophylaxis was given in accordance with guidelines from the National Blood Transfusion Service Immunoglobulin Working Party (1991). It was found that all patients received appropriate management postpartum. However, more than half of the antepartum sensitising events beyond 20 weeks' gestation were not managed according to the guidelines. Only one patient was sensitised during the studied pregnancy and no sensitising events were recorded in her case. This supports the introduction of a policy of routine anti-D prophylaxis in the third trimester.

The role of Rh antibodies in haemolytic disease of the newborn

Recent insights into the structure-function relationship of IgG, the nature of Fc receptors and their interactions with antibodies, and the cellular mechanisms involved in the immune destruction of IgG-sensitized cells have all contributed to a fuller understanding of the role of Rh antibodies in HDN. As this understanding has increased, so different diagnostic and therapeutic approaches have been developed and evaluated in order either to predict or ameliorate disease severity. The role of Rh antibodies in HDN can be considered in three contexts: maternal anti-D, monoclonal anti-D and prophylactic anti-D. Anti-D formed after maternal alloimmunization may be transported across the placenta, resulting in destruction of sensitized red cells by mononuclear phagocytes in the fetus and infant. The use of monoclonal anti-D has given an insight into the cellular and molecular mechanisms involved in red cell destruction, and has facilitated the development and evaluation of assays which use maternal anti-D to predict the severity of HDN. Polyclonal anti-D, given prophylactically, can prevent maternal alloimmunization to D-positive fetal red cells. Future developments are likely in several areas. Prophylactic polyclonal anti-D may be replaced, wholly or partially, with monoclonal anti-D. The development and introduction of cellular assays as non-invasive tests for predicting disease severity is likely to continue as preliminary results are encouraging. Finally, new strategies for ameliorating disease severity may be assessed including the role of ivIgG and Fc gamma R-blocking antibodies.

Feto-maternal haemorrhage as a cause of fetal morbidity and mortality

Massive feto-maternal haemorrhage is a cause of significant fetal morbidity and mortality, but is often overlooked because screening for fetal erythrocytes in the maternal circulation has not been a routine procedure in stillbirths, fetal distress or neonatal anaemia; there are few recognized clinical symptoms or signs, or histopathological features. This report covers three cases. A screening blood test in the last month of pregnancy could be of value in detecting feto-maternal haemorrhage before irreversible damage to the fetus occurs.

Increase in the amount of fetal lymphocytes in maternal blood during pregnancy

The presence of male fetal lymphocytes in the maternal blood of 18 pregnant women (15 primigravidae and three who had had previous pregnancies) was analysed by counting quinacrine positive Y bodies in interphases of Y chromosomes in metaphases. Counts were also performed on a control population of ten young women who had never been pregnant and on another control population of seven men to test the efficiency of the staining technique used. After completion of the calculations, comparison of the results with the sex of the newborn babies showed a correct sex prediction of 83% by interphase scoring and of 89% by metaphase scoring. The lymphocyte transfer from fetus to mother has already started by the tenth week of pregnancy, and the percentage of fetal cell present in the bloodstream of a pregnant woman increases with the duration of the pregnancy.

Birth order and intelligence: an immunological interpretation

The literature indicates that the IQs and school performance of children tend to decline with increasing order of birth. A hypothesis is here presented that the effect of birth order upon intellectual performance may result from an increasing probability of maternal immune attack upon the fetal brain in utero with order of parity. In support of this hypothesis, evidence is adduced from the literature that the fetal brain is antigenic, that fetal antigens may reach the immune system of the mother, that the incidence of maternal sensitization to fetal antigens increases with parity, that antibodies may readily cross the placenta and reach the fetal brain, that antibodies can be highly teratogenic, and that certain antibodies may damage, in a lasting way, the structure, function, and learning capacity of brains in experimental animals and human infants.

Fetal-maternal hemorrhage after amniocentesis: incidence, degree and ramifications

A review of the literature indicates a small but significant incidence of fetal-maternal hemorrhage following amniocentesis. In these studies, consideration has not been given to the frequency with which erythrocytes containing hemoglobin F are to be found in the maternal circulation in the absence of amniocentesis. In this investigation, Kleihauer-Betke analysis was carried out before and after amniocentesis. This study confirms previous reports that spontaneous fetal maternal hemorrhage is not uncommon during normal intrauterine gestation and that amniocentesis is probably only an occasional cause of fetal-maternal hemorrhage. This impression is based on the findings that most Kleihauer-positive maternal blood samples after amniocentesis were also positive before amniocentesis. In spite of these findings, and since Rh-immune serum globulin is apparently harmless to the fetus and mother, its use is still advocated in nonsensitized Rh-negative mothers after amniocentesis.

Transplacental passage of foetal blood cells

With the brightly fluorescent Y chromatin as a marker, the proportion of foetal lymphocytes in maternal blood was found to be 0.02-0.16% in 7 of 11 primigravidae bearing a boy, but not a single XY mitosis was found among 112,000 leucoagglutinin-stimulated mitoses from the same women. The proportion of foetal lymphocytes did not change appreciably during pregnancy (first appearance detected at 15 weeks' gestation), nor were there any unequivocal foetal erythrocytes in spite of ABO and Rh compatibility. These findings suggest that foetal lymphocytes pass actively into the mother's blood relatively early during pregnancy. The cells are protected from immunological elimination, and therefore may remain in the maternal blood for long periods. They are a valuable potential source of material for antenatal foetal diagnosis.

Survival of the genetically incompatible fetal allograft

A hypothesis is put forward to account for the non-rejection of the fetal allograft, based on shared surface-repellent molecules (S.R.M.s) which prevent close apposition of maternal and fetal immunocompetent cells. This hypothesis is shown to be compatible with the major theoretical requirements of the immune system. Thus it provides possible explanations for the self/not-self recognition process, tolerance to organ transplants, the development of autoimmune disease, and high and low zone tolerance. It is also argued that the recognition of self is important with reference to cellular antigens, but may not be essential for humoral antigens.

The risk of Rh isoimmunization in ruptured tubal pregnancy

In 9 (24%) out of 38 African women who had suffered a ruptured tubal pregnancy significant numbers of fetal erythrocytes (5 or more per 150,000 maternal cells) were found in the maternal circulation. This is a higher incidence than occurs after abortion and indicates that rupture of a tubal pregnancy is a potential source of Rh isoimmunization. The finding of fetal cells in the peritoneal cavity suggests that this is the main source of the fetal blood found in the maternal circulation. At operation on Rh-negative patients with ruptured tubal pregnancies, therefore, complete removal of the peritoneal blood should be attempted and the blood recovered should never be transfused into the patient, who should always receive prophylactic Rh immunoglobulin.

Immunological responses in pregnancy and survival of fetal homograft

Immunological responses were studied in pregnant women and controls using as tests phytohaemagglutinin-induced lymphocyte transformation and the tuberculin reaction. Significantly reduced responses were found to both tests in the pregnant women. These results suggest that a reduction in T-cell activity during pregnancy may help protect the fetus from rejection by its mother's immunological mechanisms.

Feto-maternal haemorrhage in therapeutic abortion

The incidence of feto-maternal haemorrhage in induced abortion has been studied with the Kleihauer technique. All four methods of termination used were shown to result in such haemorrhages. The incidence below 12 weeks' gestation, however, was very small and there seems to be no reason for offering routine rhesus-immunoglobulin to these women. When it is essential to terminate a pregnancy of 12 weeks' size or more in a rhesus-negative woman immunoglobulin should be given.

Transplacental passage of fetal red cells in abortion; increased incidence after curettage and effect of oxytocic drugs

In a study of early abortions (less than 16-week pregnancies) no significant increase in fetomaternal haemorrhage was found in patients having either threatened or incomplete abortions. A statistically significant increase in fetal cells in the maternal circulation, however, occurred after curettage. The administration of oxytocic drugs in conjunction with curettage in cases of incomplete abortion did not increase the incidence of transplacental passage of fetal erythrocytes when compared with curettage alone. Of the 81 patients curetted following abortion four had a feto-maternal haemorrhage of more than 0.2 ml. The largest amount of fetal blood found in the maternal circulation was 0.4 to 0.5 ml. Preliminary data evaluating the indirect Coombs test and enzyme-treated red cells in Rh-negative post-abortion cases suggest that this amount of blood is not a primary immunizing dose but a "booster" to preformed antibody.

Association between foeto-maternal bleeding and hypertension in pregnancy

A blind prospective survey of foeto-maternal bleeding in 200 primiparous pregnancies was carried out in an investigation of a possible association between foeto-maternal bleeding and hypertension in pregnancy. Evidence of foeto-maternal bleeding was found in 61% of 36 hypertensive pregnancies, and in 51% of 160 normotensive pregnancies, a difference which is not statistically significant.Significant differences between the hypertensive and the normotensive groups were found when foeto-maternal bleeding was related to gestation. In pregnancies that became hypertensive more foetal cells were found in the maternal circulation before week 36 than in normotensive pregnancies. In patients with oedema of the abdominal wall during pregnancy the incidence of foeto-maternal bleeding was significantly increased.These findings seem to explain why pre-eclamptic toxaemia is a significant predisposing factor in women who later develop Rh antibodies. It is recommended that anti-D gammaglobulin should be offered to all Rh-negative women with Rh-positive infants following a hypertensive pregnancy. Consideration should also be given to the question of administering anti-D gammaglobulin during Rh-negative hypertensive pregnancies if this procedure is proved to be both safe to mother and foetus and effective.The results provide contributory evidence that the placental vascular changes in toxaemic pregnancies precede the clinical signs and are not the result of hypertension.

Assessment of size of small volume feto-maternal bleeds. A new method of quantification of the Kleihauer technique

The risk of Rh-isoimmunization is probably related to the volume of foeto-maternal bleeding. With the Kleihauer technique foetal cells may be detected in maternal blood, but estimation of the size of foetal bleed the cells represent is at present difficult and open to serious error.A new method of quantifying the technique by using a standard volume of maternal blood, and a simple machine for the preparation of comparable blood films, is described. The new method eliminates many of the errors of previous techniques, and should be capable of automation for large-scale screening programmes.

Serum fibrin degradation products throughout normal pregnancy

Fibrin/fibrinogen degradation products in the serum of 169 healthy pregnant and 43 healthy non-pregnant subjects of a similar age group showed a significant increase throughout pregnancy, the highest levels being obtained in the third trimester. There was no correlation with plasma fibrinogen levels or euglobulin lysis times. Though these changes may be due to other alterations in pregnancy not primarily associated with fibrinolysis, it is possible that actual fibrinolysis is not decreased in late pregnancy, as has been previously asssumed.