question: If mother and fetus do not share blood, how do these blood tests to determine a fetuses sex work if it only collects the mother’s blood?
comment: Blood itself isn't what is used to determine gender. The fetuses DNA is in the mothers blood. So you isolate the fetus DNA, and get the gender from there.
passages: The analysis of cffDNA from a sample of maternal plasma allows for prenatal sex discernment. Applications of prenatal sex discernment include:
  
  BULLET::::- Disease testing: Whether the sex of the fetus is male or female allows the determination of the risk of a particular X-linked recessive genetic disorder in a particular pregnancy, especially where the mother is a genetic carrier of the disorder.
  
  BULLET::::- Preparation, for any sex-dependent aspects of parenting.
   Blood plasma is commonly used as test samples for verifying the maternal RhD status. Blood plasma can also be used for determining the foetal RhD status if the mother is RhD- as maternal blood plasma contains maternal DNA and trace amounts of foetal DNA. Blood samples can be obtained through venipuncture of the mother. Since plasma and other components of blood have different densities, centrifugation of blood samples with added anticoagulant (such as EDTA) can segregate blood contents into multiples layers. Blood plasma can then be isolated from the other components for rhesus factor testing. The method of extracting foetal DNA from maternal blood plasma is considered to be a type of non-invasive prenatal testing.
   After the blood samples are obtained, they are placed into tubes containing anticoagulants in order to stop the blood from clotting. If the blood sample was obtained at the site close to the placenta, a fetal blood confirmation test should be done to ensure no mixing of fetal and maternal blood occurred before the diagnostic tests are done on the blood. Fetal red blood cells (RBC) are usually bigger than maternal RBCs, and the average volume of RBCs, the mean corpuscular volume (MCV), is one of the methods used to determine whether or not the fetal blood has been contaminated. Another method, human chorionic gonadotropin (hCG) determination, can detect maternal blood because maternal blood has high levels of hCG. The hemoglobin alkaline denaturation test (Apt test) can detect the presence of maternal blood, which is indicated by a color change from red to brown when the sample is added to alkali reagent. Blood typing would also detect maternal blood, as the I antigen only occurs in adults. The Kleihauer–Betke test can detect very small amounts of maternal blood before the third trimester of pregnancy by monitoring hemoglobin elution in acid because adult and fetal hemoglobin elute differently in acid. Finally, a white blood cell count can detect maternal blood in the sample, as fetal white blood cells are primarily leukocytes, while maternal white blood cells are mostly neutrophils. If amniotic fluid infiltrated the sample, then there would be a reduction in the volume of RBCs, white blood cells, and platelets in the sample. Also, patterns consistent with amniotic fluid would be visible in the sample.
   The use of nested polymerase chain reaction (nested PCR) was evaluated to determine sex by detecting a Y chromosome specific signal in the cffDNA from maternal plasma. Nested PCR detected 53 of 55 male fetuses. The cffDNA from the plasma of 3 of 25 women with female fetuses contained the Y chromosome-specific signal. The sensitivity of nested PCR in this experiment was 96 percent. The specificity was 88 percent.
  
  Section::::Laboratory methods.:Analysis of cffDNA.:Digital PCR.
   Measurement of fetal proteins in maternal serum is a part of standard prenatal screening for fetal aneuploidy and neural tube defects. Computational predictive model shows that extensive and diverse feto-maternal protein trafficking occurs during pregnancy and can be readily detected non-invasively in maternal whole blood. This computational approach circumvented a major limitation, the abundance of maternal proteins interfering with the detection of fetal proteins, to fetal proteomic analysis of maternal blood. Entering fetal gene transcripts previously identified in maternal whole blood into a computational predictive model helped develop a comprehensive proteomic network of the term neonate. It also shows that the fetal proteins detected in pregnant woman’s blood originate from a diverse group of tissues and organs from the developing fetus. Development proteomic networks dominate the functional characterization of the predicted proteins, illustrating the potential clinical application of this technology as a way to monitor normal and abnormal fetal development.
   Tests similar to the Baby Gender Mentor test can be used for medical diagnoses. For instance, scientists have shown that it is possible to use fetal DNA from a woman's blood to screen the fetus for genetic defects such as Down syndrome. In a validation study sponsored by the National Institute for Child Health and Development, 5 different labs used fetal cells from maternal blood to search for evidence of Down syndrome in 2,744 pregnancies. On average, the labs correctly spotted Down syndrome babies 74% of the time.
   BULLET::::- Cell-free DNA can be run on certain antigens. Blood is taken from the mother, and using PCR, can detect the K, C, c, D, and E alleles of fetal DNA. This blood test is non-invasive to the fetus and is an easy way of checking antigen status and risk of HDN. Testing has proven very accurate and is routinely done in the UK at the International Blood Group Reference Laboratory in Bristol. Sanquin laboratory in Amsterdam, Netherlands also performs this test. For US patients, blood may be sent to either of the labs. In the US, Sensigene is done by Sequenome to determine fetal D status. Sequenome does not accept insurance in the US, but US and Canadian patients have had insurance cover the testing done overseas.
   Rh disease sensitization is about 10 times more likely to occur if the fetus is ABO compatible with the mother than if the mother and fetus are ABO incompatible.
  
  Section::::Diagnosis.
  
  BULLET::::- Maternal blood
  
  BULLET::::- The Kleihauer–Betke test or flow cytometry on a postnatal maternal blood sample can confirm that fetal blood has passed into the maternal circulation and can also be used to estimate the amount of fetal blood that has passed into the maternal circulation.
   The earliest post-implantation test, cell free fetal DNA testing, involves taking a blood sample from the mother and isolating the small amount of fetal DNA that can be found within it. When performed after week seven of pregnancy, this method is about 98% accurate.
   A maternal peripheral blood sample is taken by venesection at about ten weeks gestation.
  
  Section::::Laboratory methods.:Separation of cffDNA.
  
  Blood plasma is separated from the maternal blood sample using a laboratory centrifuge. The cffDNA is then isolated and purified. A standardized protocol for doing this was written through an evaluation of the scientific literature. The highest yield in cffDNA extraction was obtained with the "QIAamp DSP Virus Kit".
   BULLET::::- Cell-free DNA can be run on certain antigens. Blood is taken from the mother, and using PCR, can detect the K, C, c, D, and E alleles of fetal DNA. This blood test is non-invasive to the fetus and is an easy way of checking antigen status and risk of HDN. Testing has proven very accurate and is routinely done in the UK at the International Blood Group Reference Laboratory in Bristol. Sanequin laboratory in Amsterdam, Netherlands also performs this test. For US patients, blood may be sent to either of the labs. In the US, Sensigene is done by Sequenome to determine fetal D status. Sequenome does not accept insurance in the US, but US and Canadian patients have had insurance cover the testing done overseas.
   Routine determination of fetal RhD status from cffDNA in maternal serum allows early management of at risk pregnancies while decreasing unnecessary use of Anti-D by over 25 percent.
  
  Section::::Applications.:Aneuploidy.
  
  BULLET::::- Sex chromosomes
  
  Analysis of maternal serum cffDNA by high-throughput sequencing can detect common fetal sex chromosome aneuploidies such as Turner's syndrome, Klinefelter's syndrome and triple X syndrome but the procedure's positive predictive value is low.
  
  BULLET::::- Trisomy 21
   BULLET::::- Cell-free DNA can be run on certain antigens. Blood is taken from the mother, and using PCR, can detect the K, C, c, D, and E alleles of fetal DNA. This blood test is non-invasive to the fetus and is an easy way of checking antigen status and risk of HDN. Testing has proven very accurate and is routinely done in the UK at the International Blood Group Reference Laboratory in Bristol. Sanequin laboratory in Amsterdam, Netherlands also performs this test. For US patients, blood may be sent to either of the labs. In the US, Sensigene is done by Sequenome to determine fetal D status. Sequenome does not accept insurance in the US, but US and Canadian patients have had insurance cover the testing done overseas.
   Section::::Fetal red-blood-cell detection problems.
  
  Since fetal and maternal blood cells have the same life expectancy in the maternal bloodstream, it is possible to obtain informative results from a KB stain for a fair period of time after a stillbirth. However, if the mother and fetus are ABO incompatible, it is more crucial to quickly perform the KB stain following a stillbirth, as the fetal red blood cells will be eliminated from the maternal bloodstream very quickly, causing the KB stain to underestimate the degree of FMH, if any.
   BULLET::::- The serum test, using 2-4 mL of venous blood, is typically a chemiluminescent or fluorimetric immunoassay that can detect βhCG levels as low as 5 mIU/ml and allows quantification of the βhCG concentration.
  
  Section::::Testing.:Reference levels in normal pregnancy.
  
  The following is a list of serum hCG levels. ("LMP" is the last menstrual period dated from the first day of the last menstrual period.) The levels grow exponentially after conception and implantation.
  
  Section::::Testing.:Interpretation.
   BULLET::::- Cell-free DNA can be run on certain antigens. Blood is taken from the mother, and using PCR, can detect the K, C, c, D, and E alleles of fetal DNA. This blood test is non-invasive to the fetus and is an easy way of checking antigen status and risk of HDN. Testing has proven very accurate and is routinely done in the UK at the International Blood Group Reference Laboratory in Bristol. Sanequin laboratory in Amsterdam, Netherlands also performs this test. For US patients, blood may be sent to either of the labs. In the US, Sensigene is done by Sequenome to determine fetal D status. Sequenome does not accept insurance in the US, but US and Canadian patients have had insurance cover the testing done overseas.
   Section::::Conditions screened.
  
  The most common abnormality the test can screen is trisomy 21 (Down syndrome). In addition to Down syndrome, the triple and quadruple screens assess risk for fetal trisomy 18 also known as Edward's syndrome, open neural tube defects, and may also detect an increased risk of Turner syndrome, triploidy, trisomy 16 mosaicism, fetal death, Smith–Lemli–Opitz syndrome, and steroid sulfatase deficiency.
  
  Section::::Values measured.
  
  The triple test measures the following three levels in the maternal serum:
  
  BULLET::::- alpha-fetoprotein (AFP)
  
  BULLET::::- human chorionic gonadotropin (hCG)
  
  BULLET::::- unconjugated estriol (UE)
  
  Section::::Interpretation.
   Section::::Prenatal paternity testing for unborn child.:Non-invasive prenatal paternity testing.
  
  Current advances in genetic testing have led to the ability to determine who the biological father is while the woman is still pregnant through a non-invasive method. There is a small amount of fetal DNA (cffDNA) present in the mother's blood during pregnancy. This allows for accurate fetal DNA paternity testing during pregnancy from a blood draw with no risk of miscarriage. Studies have shown that cffDNA can first be observed as early as 7 weeks gestation, and the amount of cffDNA increases as the pregnancy progresses.
  
  Section::::DNA profiling.
   The test is easily performed and is usually painless. A specimen is collected from the patient using a vaginal swab. The swab is placed in a transport tube and sent to a laboratory for testing. Most labs can easily produce a result in less than one hour.
  
  A false positive fetal fibronectin result can occur if the test is performed after digital examination of the cervix or after having had intercourse. It is important that the swab be taken before a digital vaginal exam is performed.
  
  Section::::See also.
  
  BULLET::::- Fibronectin
  
  BULLET::::- Fibronectin type II domain
  
  BULLET::::- Premature labor
  
  Section::::External links.
   High throughput shotgun sequencing using tools such as Solexa or Illumina, yields approximately 5 million sequence tags per sample of maternal serum. Aneuploid pregnancies such as trisomy where identified when testing at the fourteenth week of gestation. Fetal whole of genome mapping by parental haplotype analysis was completed using sequencing of cffDNA from maternal serum. Pregnant females were studied using a 2-plex massively parallel maternal plasma DNA sequencing and trisomy was diagnosed with z-score greater than 3. The sequencing gave sensitivity of 100 percent, specificity of 97.9 percent, a positive predictive value of 96.6 percent and a negative predictive value of 100 percent.
   BULLET::::- Sex selection, which after preimplantation genetic diagnosis may be performed by selecting only embryos of the preferred sex, or, after post-implantation methods by performing sex-selective abortion depending on the test result and personal preference.
  
  In comparison to obstetric ultrasonography which is unreliable for sex determination in the first trimester and amniocentesis which carries a small risk of miscarriage, sampling of maternal plasma for analysis of cffDNA is without risk. The main targets in the cffDNA analysis are the gene responsible for the sex-determining region Y protein (SRY) on the Y chromosome and the DYS14 sequence.
  
  Section::::Applications.:Congenital adrenal hyperplasia.
   BULLET::::- Cell-free fetal DNA testing, wherein a venipuncture is performed on the mother to analyze the small amount of fetal DNA that can be found within it. It provides the earliest post-implantation test. A meta-analysis published in 2011 found that such tests are reliable more than 98% of the time, as long as they are taken after the seventh week of pregnancy.
   If analysis of cffDNA obtained from a sample of maternal plasma lacks genetic markers found only on the Y chromosome, it is suggestive of a female fetus. However, it might also indicate a failure of the analysis itself ( a false negative result). Paternal genetic polymorphisms and sex-independent markers may be used to detect cffDNA. An high degree of heterozygosity of these markers must be present for this application.
  
  Section::::Applications.:Paternity testing.
  
  Prenatal DNA paternity testing is commercially available. The test can be performed at nine weeks gestation.
  
  Section::::Applications.:Single gene disorders.
   sex-determining region Y gene (SRY) and Y chromosome short tandem repeat "DYS14" in cffDNA from 511 pregnancies were analyzed using quantitative real-time PCR (RT-qPCR). In 401 of 403 pregnancies where maternal blood was drawn at seven weeks gestation or more, both segments of DNA were found.
  
  Section::::Laboratory methods.:Analysis of cffDNA.:Nested PCR.
   Obstetric ultrasonography, either transvaginally or transabdominally, checks for various markers of fetal sex. It can be performed at or after week 12 of pregnancy. At this point, of fetal sexes can be correctly determined, according to a 2001 study. Accuracy for males is approximately 50% and for females almost 100%. When performed after week 13 of pregnancy, ultrasonography gives an accurate result in almost 100% of cases.
  
presuppositions: Blood of the mother determines the sex of a child. Blood of the mother determines the sex of a child.
corrections: It is not the blood itself that determines the child's sex, it is the DNA of the fetus that is isolated from the DNA inside the mothers blood that really determines the outcome. It is not the blood itself that determines the child's sex, it is the DNA of the fetus that is isolated from the DNA inside the mothers blood that really determines the outcome.
raw_presuppositions: Blood of the mother determines the sex of a child. Blood of the mother determines the sex of a child. Blood of the mother determines the sex of a child.
raw_corrections: It is not the blood itself that determines the child's sex, it is the DNA of the fetus that is isolated from the DNA inside the mothers blood that really determines the outcome. It is not the blood itself that determines the child's sex, it is the DNA of the fetus that is isolated from the DNA inside the mothers blood that really determines the outcome. It is not the blood itself that determines the child's sex, it is the DNA of the fetus that is isolated from the DNA inside the mothers blood that really determines the outcome.