Analysis of peripheral maternal blood samples for the presence of placenta-derived cells using Y-specific probes and McAb H315

Post-genomics studies and their application to non-invasive prenatal diagnosis

Non-invasive prenatal diagnosis (NIPD) offers the opportunity to eliminate completely the risky procedures of amniocentesis and chorionic villus sampling. The development of NIPD tests has largely centred around the isolation and analysis of fetal cells in the maternal circulation and the analysis of free fetal DNA in maternal plasma. Both of these techniques offer difficult technical challenges, and at the current moment in time the use of free fetal DNA is the simplest and most effective method of defining paternally inherited fetal genes for diagnosis. Post-genomics technologies that explore the proteins (proteomics) and transcripts (transcriptomics) released by the placenta into the maternal circulation offer new opportunities to identify genes and their protein products that are key diagnostic markers of disease (in particular Down syndrome), and might replace the current screening markers in use for prediction of risk of Down syndrome. In the ideal situation, these markers are sufficiently diagnostic not to require invasive sampling of fetal genetic material. Post-genomics techniques might also offer better opportunities for defining fetal cell-specific markers that might enhance their isolation from maternal blood samples. This review describes progress in these studies, particularly those funded by the Special Non-invasive Advances in Fetal and Neonatal Evaluation (SAFE) Network of Excellence.

Measurement of mRNA of trophoblast-specific genes in cellular and plasma components of maternal blood

BACKGROUND

Placental mRNA in maternal plasma is suitable for quantitative analysis regardless of fetal gender and genetic polymorphism status.

METHODS

We obtained 155 blood samples from pregnant women to compare human placental lactogen (hPL) and beta-subunit of human chorionic gonadotropin (beta hCG) mRNA and protein levels between the cellular and plasma components of maternal blood. To assess clearance of hPL mRNA expression, we obtained blood samples from nine women immediately before and after delivery by caesarean section. mRNA was extracted from the cellular and plasma components of all samples, and hPL and beta hCG mRNA expression was analysed by reverse transcription-PCR assay.

RESULTS

The concentration of beta hCG mRNA in the cellular component positively correlated with the plasma concentration of beta hCG protein and beta hCG mRNA (p = 0.001 for both). The concentration of hPL protein in the plasma correlated with the hPL mRNA concentration of the cellular component (p<0.05). For both hPL and beta hCG, the mRNA concentration of the cellular component was greater than that of the plasma component (22.9-fold higher for hPL and 4.3-fold higher for beta hCG). The half life of hPL mRNA clearance was significantly longer for the cellular fraction (mean half life = 203.8 min, range 150-3465 min) than for the plasma fraction (mean half life = 32.2 min, range 15-385 min) (p = 0.008).

CONCLUSION

The present findings indicate that the concentration of hPL and beta hCG mRNA is significantly higher in the cellular component of maternal blood samples than in the plasma component. Cellular mRNA in maternal blood is useful for non-invasive evaluation of placental function.

Clinical Potential for Noninvasive Prenatal Diagnosis Through Detection of Fetal Cells in Maternal Blood

Fetal cells circulate in maternal blood and are considered a suitable means by which to detect fetal genetic and chromosomal abnormalities. This approach has the advantage of being noninvasive. Since the early 1990s, nucleated erythrocytes (NRBCs) have been considered good target cells for a number of techniques, including fluorescence-activated cell sorting and magnetic cell sorting, using antibodies such as anti-transferrin receptor and anti-gamma-hemoglobin antibodies, followed by analysis with fluorescence in situ hybridization or polymerase chain reaction. In the late 1990s, the National Institute of Child Health and Human Development Fetal Cell Isolation Study assessed the reliability of noninvasive prenatal diagnosis of fetal aneuploidy using NRBCs isolated from maternal circulation. This study revealed the limitations of NRBC separation using antibodies specific for NRBC antigens. A more recent study has demonstrated the efficiency and success of recovery of NRBCs using a galactose-specific lectin, based on the observation that erythroid precursor cells have a large quantity of galactose molecules on their cell surface. Thus, recent advances in this field enhance the feasibility of this diagnostic method. This review article focuses on various methods of detection of fetal cells within the maternal circulation, as well as the status of previous and current studies and the prospective view for noninvasive prenatal diagnosis using fetal cells from the maternal circulation.

Long-term feto-maternal microchimerism: nature's hidden clue for alternative donor hematopoietic cell transplantation?

During pregnancy, fetal hematopoietic cells carrying paternal human leukocyte antigens (HLA) migrate into maternal circulation, and, vice versa, maternal nucleated cells can be detected in fetal organs and umbilical cord blood, indicating the presence of bidirectional cell traffic between mother and fetus. By taking advantage of fluorescence in-situ hybridization or polymerase chain reaction-based techniques, researchers recently found that postpartum persistence of such reciprocal chimerism was common among healthy individuals and may sometimes cause tissue chimerism. Although the biological significance of long-lasting feto-maternal microchimerism is unknown, a number of investigations have suggested its association with the development of "autoimmune" diseases such as systemic sclerosis. However, the very common presence of feto-maternal microchimerism among subjects without any autoimmune attack may allow us the more appealing hypothesis that it is an indicator for the acquired immunological hyporesponsiveness to noninherited maternal or fetal HLA antigens. An offspring's tolerance to noninherited maternal antigens has been clinically suggested by the retrospective analysis of renal transplantations or haploidentical hematopoietic stem cell transplantations, and whether postpartum mothers can tolerate paternally derived fetal antigens is an intriguing question. Although an exact linkage between microchimerism and transplantation tolerance is yet to be elucidated, long-term acceptance of a recipient's cell in the donor may have a favorable effect on preventing the development of severe graft-versus-host disease, and the donor cell microchimerism in the recipient might facilitate the graft acceptance. If this concept holds true, HLA-mismatched hematopoietic stem cell transplantation would be more feasible among haploidentical family members mutually linked with feto-maternal microchimerism. Further studies are warranted to investigate the potential role of feto-maternal microchimerism in human transplantation medicine.

First trimester prenatal diagnosis: fetal cells in the maternal circulation

The recovery of fetal cells from the maternal circulation represents a promising approach to noninvasive prenatal diagnosis. Advances in techniques of sensitive molecular genetic analysis have enabled the conclusive demonstration of the presence of fetal cells in maternal blood. In most pregnancies, there are few fetal cells detectable. In some abnormal pregnancies, there appears to be increased fetomaternal transfusion, which facilitates recognition of aneuploid fetal cells. This review article describes general strategies of fetal cell isolation, current technical challenges, and clinical applications that are envisioned for the future. The increased appreciation of fetal cell microchimerism, and its association with complications of pregnancy and the postpartum development of autoimmune disease, is also discussed.

The application of individualised fetal growth curves

Individually adjusted or 'customised' growth charts aim to optimise the assessment of fetal growth by taking individual variation into account, and by projecting an optimal curve which delineates the potential weight gain in each pregnancy. This results in an increased detection rate of true growth restriction and a reduction in false positive diagnoses for IUGR. An adjustable standard can apply across geographical boundaries, as individual variation exceeds that between different maternity populations.

Prenatal diagnosis using fetal cells from the maternal circulation

Current prenatal diagnosis relies on invasive methods such as amniocentesis and chorionic villus sampling. Because these methods carry a low, but finite risk of pregnancy loss, noninvasive genetic screening techniques are the focus of intense research. Isolating fetal cells from maternal blood for genetic analysis is the least invasive method currently being investigated. We discuss the various methods that have been used to isolate these cells. Nucleated red blood cells have emerged as the ideal fetal cell type. This is because they have the DNA material necessary for genetic analysis, they are consistently present in maternal blood, they can be easily identified based on their morphology, and they have a definite gestational life span.

Prenatal diagnosis by analysis of fetal cells in maternal blood

The data accumulated thus far indicate that fetal NRBCs are the target cell type of choice in maternal blood for most investigators, although some groups continue to work with the trophoblast. Reports of persistent circulation of hematopoietic stem cells, lymphoid/myeloid progenitors, and lymphocytes mandate that removal of these cell types must occur before clinical diagnosis of the current pregnancy can be made. In selected cases, accurate detection of fetal aneuploidy has been made from fetal cells in maternal blood; the clinical evaluation sponsored by the National Institute of Child Health and Human Development will determine the sensitivity and specificity of cytogenetic diagnosis in a larger group of pregnant women, but this information will not be available for several years. At present, detection of uniquely fetal, paternally inherited gene polymorphisms or mutations such as the Rh(D) antigen is possible only because the mother lacks these genes; hence, maternal cell contamination does not hinder diagnosis. Currently the presence of large numbers of maternal cells in enriched samples precludes single-gene diagnosis for conditions in which the mother carries a mutant gene, because her cells are preferentially amplified and difficult to distinguish from those of the fetus. It is likely, however, that as techniques of individual fetal cell isolation are perfected, maternal cell contamination will no longer be an issue, and the entire fetal genome will become available for diagnosis and therapy. Pediatricians need to be aware of the progress of research in this field, because fetal cell isolation from maternal blood not only could change prenatal diagnosis but would change the amount of genetic information that arrives with a newborn infant at birth. The ultimate goal of this work is to diagnose noninvasively, in the first trimester, the common fetal aneuploidies and single-gene disorders, to permit in utero treatment, or to allow low-risk pregnant women carrying an abnormal fetus an opportunity for reproductive choice.

Enrichment of fetal nucleated cells from maternal blood: model test system using cord blood

The presence of small numbers of fetal nucleated red cells in the maternal circulation has been a stimulus for the development of technologies for non-invasive prenatal genetic analysis. Our laboratory has been assessing the feasibility of density gradient centrifugation followed by magnetic activated cell sorting (MACS) of cells expressing CD32 and CD45, to deplete maternal nucleated blood cells. We have examined the efficiency of each of the steps of this procedure using cord blood from term pregnancies as a source of nucleated red blood cells. Cord blood was shown to contain highly variable numbers of nucleated red cells. Three different density gradients were examined. There was no major difference in the performances of the double and triple gradients. Density gradient centrifugation resulted in enrichments of nucleated red blood cells of about 1000-fold relative to the total cell count. However, it was apparent that the selection of the cell layers which were most enriched for these cells would result in significant losses of nucleated red cells in other layers. MACS sorting of cells using CD45 resulted in white cell depletions ranging from 7 to 34-fold. These data provide a foundation for comparison with other methods and for optimization of the MACS technique.

Enrichment of fetal cells from maternal blood by high gradient magnetic cell sorting (double MACS) for PCR-based genetic analysis

For simple and effective isolation of fetal cells from peripheral maternal blood, we combined depletion of maternal cells and enrichment of fetal cells by high-gradient magnetic cell separation (MACS). First CD45+ and CD14+ cells were depleted from maternal peripheral blood mononuclear cells by MACS. From the depleted fraction, CD71+ erythroid cells were enriched up to 80 per cent by MACS. This double-MACS' procedure yielded an average depletion rate of 780-fold and an average enrichment rate of 500-fold, with approximate recovery rates of 40-55 per cent. For paternity testing, cells from unseparated blood and the various fractions were analysed for polymorphism of the HLA-DQ-A1 locus and D1S80 locus by the polymerase chain reaction (PCR). In CD45-/CD71+ sorted cells from maternal blood, but not in unfractionated cells from maternal blood or CD45-/CD14- cells, paternal alleles could be detected. In the CD45-/CD71+ fraction, the relative frequency of paternal alleles compared with maternal alleles ranged from 1 in 20 to 1 in 200 (determined by titration and depending on the quality of separation and biological variation). In 7 out of 11 cases, between weeks 12 and 25 of gestation, we could identify paternal alleles by PCR, either HLA-DQ-A1 or D1S80. This double-MACS procedure is simple, fast, efficient, and reliable for non-invasive prenatal diagnosis.

Isolating fetal cells in maternal circulation for prenatal diagnosis

Fetal cells unequivocally exist in and can be isolated from maternal blood. Erythroblasts, trophoblasts, granulocytes and lymphocytes have all been isolated by various density gradient and flow sorting techniques. Chromosomal abnormalities detected on isolated fetal cells include trisomy 21, trisomy 18, Klinefelter syndrome (47,XXY) and 47,XYY. Polymerase chain reaction (PCR) technology has enabled the detection of fetal sex, Mendelian disorders (e.g. beta-globin mutations), HLA polymorphisms, and fetal Rhesus (D) blood type. The fetal cell type that has generated the most success is the nucleated erythrocyte; however, trophoblasts, lymphocytes and granulocytes are also considered to be present in maternal blood. Fetal cells circulate in maternal blood during the first and second trimesters, and their detection is probably not affected by Rh or ABO maternal-fetal incompatibilities. Emphasis is now directed toward determining the most practical and efficacious manner for this technique to be applied to prenatal genetic diagnosis. Only upon completion of clinical evaluations could it be considered appropriate to offer this technology as an alternative to conventional invasive and non-invasive methods of prenatal cytogenetic diagnosis.

A morphologic study of trophoblast isolated from peripheral blood of pregnant women

OBJECTIVE

Our purpose was to evaluate the morphologic characteristics of cells separated from peripheral maternal blood with immunomagnetic beads coated with trophoblast-reactive monoclonal antibodies of restricted specificity.

STUDY DESIGN

Blood samples were collected from 14 pregnant women at 9 to 12 or 16 to 18 weeks' gestation. Immunomagnetic beads coated with trophoblast-reactive monoclonal antibodies were used to isolate cells from the blood. The isolated cells were then fixed, embedded, and sectioned to enable morphologic identification.

RESULTS

Multinucleate cells identical to syncytiotrophoblast sprouts were identified in 12 of 14 samples.

CONCLUSION

Multinucleate syncytiotrophoblast can be isolated from the peripheral blood of normal pregnant women in both the first and second trimesters of pregnancy. The isolated trophoblast may allow prenatal genetic diagnosis by a noninvasive technique.

Erythroid-specific antibodies enhance detection of fetal nucleated erythrocytes in maternal blood

Fetal nucleated erythrocytes (NRBC) in maternal blood are a non-invasive source of fetal DNA for prenatal genetic screening. We compared the effectiveness of three monoclonal antibodies for the separation of fetal cells from maternal blood by flow sorting. Mononuclear blood cells from 49 healthy pregnant women were incubated with antibody to CD 71, CD 36, and/or glycophorin A (GPA), employed singly or in combination with each other. These monoclonal antibodies recognize surface antigens on haematopoietic precursor cells. Successful isolation of fetal cells was defined as detection of Y chromosomal sequences in maternal blood from women carrying male fetuses, with absence of Y sequences when female fetuses were carried. Thus, gender prediction accuracy was used as a measure of fetal cell separation. Using anti-CD 71 to isolate fetal cells, gender prediction was 57 per cent correct; with anti-CD 36, it was 88 per cent correct. Anti-GPA, an erythrocyte-specific antigen, used alone or in combination with anti-CD 71 or 36, improved gender prediction to 100 per cent. We conclude that antibody to GPA improves the retrieval of fetal NRBC from maternal blood, permitting genetic analysis by the polymerase chain reaction.

Fetal granulocytes in maternal venous blood detected by in situ hybridization

Fetal male cells from maternal venous blood were detected by a non-radioactive in situ hybridization method using the biotinylated Y-specific DNA probe pY431. The hybridizations were performed on Ficoll-Paque-isolated nucleated blood cells obtained from 11 pregnant women in the seventh to 31st week of gestation. A Y-specific signal was detected in both granulocytes and lymphocyte-like cells in seven of the 11 women studied. These women gave birth to boys. In one of the four remaining cases, a Y-specific signal was detected in the lymphocyte-like cells but not in the granulocytes. This woman gave birth to a girl. The other three women had no cells with a Y-specific signal and all three gave birth to girls. Altogether, 83,500 nucleated cells were analysed. One hundred and three cells showed a Y-specific signal. Of these Y-specific cells, 62 per cent were granulocytes and 38 per cent lymphocyte-like cells. Our results suggest that fetomaternal transfer of granulocytes is common and that it occurs as early as in the seventh week of gestation. None of the ten non-pregnant female control samples showed positive cells with the Y-chromosome-specific probe; approximately 97 per cent of the cells from the five adult male controls showed a Y-specific signal. Our results indicate that in situ hybridization using a Y-specific DNA probe performed on granulocytes in maternal blood can be used for fetal male sex determination.

Molecular cytogenetics: Diagnosis and prognostic assessment

This review describes molecular cytogenetic techniques for detection and characterization of genetic aberrations associated with human disease. The techniques of fluorescence in situ hybridization, primed in situ labeling and comparative genome hybridization are described, as are probes for repeated sequences, whole chromosomes and specific loci. Also reviewed are applications of these technologies to pre- and neonatal diagnosis and to the characterization of human malignancies.

Extravillus dividing fetal cells at CVS: evidence of their erythroblastic origin

Cytological and cytogenetic studies were performed on nucleated fetal cells present in chorionic villus transport medium. The erythroblastic origin of these cells was shown. Fetal erythroblasts in spontaneous mitosis were frequently observed; chromosome counts were obtained from them but poor quality often prevented banded analysis. Cytogenetic study of erythroblast metaphases can be useful as an additional diagnostic aid in cases of mosaicism with aneuploid cell lines.

The normal fetomaternal immune relationship

The antigenic status of the preimplantation embryo is ill-defined and there are no clearly recognized maternal immune reactions against this early stage of development. Following implantation, the pregnant female shows evidence of immune recognition of her intrauterine allogeneic conceptus. In a proportion of pregnancies, particularly in multiparous women, there are maternal cytotoxic antibodies exhibiting specificity for the paternally inherited HLA antigens of the fetus. When these are undetectable there may be other antibodies that are non-complement fixing and non-cytotoxic or antibodies that are not present as free molecules and incapable of identification in conventional assays. Anti-HLA antibodies pose no threat to the fetus, principally owing to their absorption by the placenta and, very likely, the harmless binding of any that do reach the fetal circulation. No potentially deleterious cytotoxic T lymphocyte generation occurs in most pregnancies. The extent to which this is due to maternal immunoregulatory control processes is not yet established. The fetal trophoblast is able to act as a protective barrier by virtue of unique properties, including a lack of conventional class I and class II HLA molecules, that render it insusceptible to immune attack. The nature and significance of any maternal recognition of non-HLA antigens on trophoblast await elucidation. Maternal immune cell traffic across the placenta occurs only at a very low level, if at all, in normal pregnancy. This may take place to a greater degree in some of the rare instances of fetal graft-versus-host disease, but this is complicated by the associated fetal immunodeficiency. Maternal IgG antibodies are transmitted across the placental trophoblast by receptor-dependent mechanisms to provide immediate protection for the neonate against environmental pathogens. Leakage of fetal erythrocytes, leukocytes and platelets into the maternal circulation can elicit IgG isoantibodies that take advantage of the same mechanisms to gain access to the fetus, with pathological consequences. Autoantibodies in women with various disease states may similarly pass into the fetus but these normally produce only mild and transient effects. The development of the fetal immune system begins at an early stage of gestation. It is competent to respond to intrauterine infections from as early as 12 weeks and has full functional potential at birth. Maternally acquired IgG is available for up to 9 months of life until the infant's own immune system has been adequately primed and activated following first exposure to specific antigens. The normal fetomaternal immune relationship represents a remarkable harmonious association between two genetically disparate individuals.(ABSTRACT TRUNCATED AT 400 WORDS)

Magnetic cell sorting and the transferrin receptor as potential means of prenatal diagnosis from maternal blood

OBJECTIVE

We wanted to test whether the recently described method of using the transferrin receptor system for fluorescence-activated cell-sorter enrichment of nucleated red blood cells can be used for prenatal diagnosis from maternal blood.

STUDY DESIGN

Instead of the laborious, expensive fluorescence-activated cell-sorter system, we used the newly described magnetic-activated cell sorter.

RESULTS

An effective enrichment could be achieved with separation of lymphocyte subsets. With the transferrin receptor, however, the enrichment was very inefficient because of the poor specificity of the antibody itself. Even in umbilical cord blood only 25% of nucleated red blood cells were labeled as demonstrated by immunogold silver enhancement of transferrin receptor-labeled cells.

CONCLUSION

In spite of the availability of a fast and effective separation method (magnetic-activated cell sorter) the use of the transferrin receptor antigen alone is not likely to enable a reliable identification of fetal cells in maternal circulation.

Trophoblast-like cells sorted from peripheral maternal blood using flow cytometry: a multiparametric study involving transmission electron microscopy and fetal DNA amplification

Three monoclonal antibodies (MAbs) against trophoblast (GB17, GB21, and GB25) and flow cytometry were used to sort trophoblast-like cells (TLCs) from peripheral blood of pregnant women. Sorted TLCs were processed for electron microscopy and fetal DNA amplification of the Y-specific sequences from mothers carrying male fetuses. At the ultra-structural level, most of the nucleated cells had the morphology of leucocytes, suggesting maternal contaminants, and we did not find the characteristic features of the free intervillous trophoblast cells. Nevertheless, polymerase chain reaction (PCR) analysis showed an amplification of Y-specific sequences in two out of three samples of sorted TLCs. These results suggest that besides the maternal leucocytes, sufficient trophoblast nucleated fetal cells can be obtained using cell enrichment by sorting. This sensitive method holds promise for non-invasive prenatal diagnosis of fetal sex and if sufficient Y(positive) nuclei are found, for the diagnosis of selected numerical chromosome abnormalities.

Detection of fetal cells in maternal blood

We report the detection of fetal cells in the maternal circulation by enzymatic amplification of a single copy gene sequence that was fetal-specific. Fetal HLA-A2-positive cells were sorted from maternal HLA-A2-negative cells by flow cytometry and confirmed by demonstration of a fetal-specific HLA-DR4 sequence. However, this sequence could not be detected in unenriched maternal DNA prepared at 28 and 32 weeks' gestation. The sensitivity of detection was 1 HLA-DR4-positive cell in 10(5) HLA-DR4-negative cells. We conclude that prenatal diagnosis of paternally inherited autosomal-dominant genetic defects may be possible by selective gene amplification of maternal peripheral blood. However, preliminary enrichment for fetal cells may be necessary.

Ratio of fetal to maternal DNA is less than 1 in 5000 at different gestational ages in maternal blood

Using Southern hybridization with the DNA probe pY3.4, we were not able to detect fetal DNA in blood of 36 pregnant women carrying male fetuses. Gestational ages ranged from 8-40 weeks of pregnancy. Using the same DNA probe, we were able to detect the male-specific signal in experimental dilution series down to 1/5000 on autoradiograms. We conclude that the ratio of fetal DNA in maternal circulation, in contrast to previous estimations, must be lower than 1/5000.

Isolation of fetal DNA from nucleated erythrocytes in maternal blood

Fetal nucleated cells within maternal blood represent a potential source of fetal genes obtainable by venipuncture. We used monoclonal antibody against the transferrin receptor (TfR) to identify nucleated erythrocytes in the peripheral blood of pregnant women. Candidate fetal cells from 19 pregnancies were isolated by flow sorting at 12 1/2-17 weeks gestation. The DNA in these cells was amplified for a 222-base-pair (bp) sequence present on the short arm of the Y chromosome as proof that the cells were derived from the fetus. The amplified DNA was compared with standardized DNA concentrations; 0.1-1 ng of fetal DNA was obtained in the 20-ml maternal samples. In 7/19 cases, a 222-bp band of amplified DNA was detected, consistent with the presence of male DNA in the isolated cells; 6/7 of these were confirmed as male pregnancies by karyotyping amniocytes. In the case of the female fetus, DNA prepared from samples at 32 weeks of gestation and cord blood at delivery also showed the presence of the Y chromosomal sequence, suggesting Y sequence mosaicism or translocation. In 10/12 cases where the 222-bp band was absent, the fetuses were female. Thus, we were successful in detecting the Y chromosomal sequence in 75% of the male-bearing pregnancies, demonstrating that it is possible to isolate fetal gene sequences from cells in maternal blood. Further refinement in methodology should increase sensitivity and facilitate noninvasive screening for fetal gene mutations.

Prenatal sex determination by DNA amplification from maternal peripheral blood

The polymerase chain reaction was used to amplify a Y-specific repeat sequence in peripheral blood DNA samples from 19 pregnant women who had a gestational age of 9 to 41 weeks. Y-specific sequences were amplified from all 12 women who bore a male fetus but in none of 7 women who bore a female fetus. With stringent precautions against contamination, this technique may assist prenatal diagnosis of sex-linked genetic disorders.

Attempts to identify trophoblast in peripheral blood during pregnancy using monoclonal antibody H315

In view of its potential importance in antenatal diagnosis of genetic disease, we have used flow cytometry to attempt to enumerate trophoblast cells in the peripheral blood of pregnant females. As a trophoblast marker we have used the monoclonal antibody (MAB) H315, which reacts with placental-type alkaline phosphatase (PLAP) associated with the cell-membrane of the syncytiotrophoblast. We analyzed blood leucocytes from 62 pregnant females between 6 and 22 weeks' gestation, 26 nonpregnant females, and 21 males. Overall, we detected more H315+ cells in pregnant than in nonpregnant female blood but, because of the wide variation in both groups, we found no statistically significant difference between them. We were unable to detect H315+ cells in 25% of pregnant females within the resolution of the technique. There was, in contrast, a significant difference between the number of H315+ cells in pregnant females and males. Because we were unable to demonstrate other trophoblast markers on H315+ cells isolated from maternal peripheral blood, the origin of these cells is unclear. This implies that these cells may be an unsuitable source of material for antenatal diagnosis.