Prenatal Paternity Test


  • It is possible to conduct a prenatal paternity test before the birth of a child.
  • The main challenge is how to obtain a suitable DNA sample from the baby – there are two types of approach:
  • The first approach requires invasive sampling either by drawing fetal amniotic fluid from inside the uterus (amniocentesis) or by taking fetal placental tissue via Chorionic Villus Sampling (CVS).
  • CVS can generally be used between weeks 10 and 14 of pregnancy and amniocentesis after 14 weeks.
  • Invasive sampling procedures carry a small increased risk of miscarriage.
  • The main advantage of invasive sampling is that the powerful STR paternity testing method can be used.
  • The second approach is non-invasive and relies on the presence of a small amount of fetal DNA normally found in mother’s bloodstream.
  • There is no additional risk to the baby as only a sample of mother’s blood is needed for comparison with the questioned father’s sample.
  • Maternal blood samples are taken after week 10 and best tested by a relatively new but reliable method involving SNPs.
  • The SNP method is not yet accepted for use in legal decision making and is generally only used for “peace-of-mind” tests. But, bear in mind, that it is always possible to conduct an STR test after birth to confirm the prenatal SNP result and to resolve any legal issues.
Read on for a more detailed explanation

Silhouette preg woman & partner When paternity of an unborn child is uncertain it is often desirable to establish who is the biological father before the child is born. Everybody’s circumstances are different but there are many reasons why it might be important to be certain as early in pregnancy as possible, including the following: Piece of mind for both parents; greater likelihood of emotional support from the father; more time to build an emotionally stable environment for the baby; clarity about whose name is to go on the birth certificate; early clarity on implications such as financial support and entitlement to legal and social benefits.

It is notoriously difficult to calculate an accurate date on which conception occurred and if pregnancy begun at around a time when there has been more than one sexual partner it can be difficult to be sure which act of intercourse caused the pregnancy. Estimates of the conception date have a sizeable margin of error because they are based on menstrual cycle timings and estimates of ovulation times when an egg is available to be fertilized. Unfortunately, even with women whose periods are regular, there are variations in cycle and ovulation times. Furthermore, if you add the fact that semen can remain inside a woman’s reproductive system for several days after intercourse you can see why it is difficult to be sure who is the father, even if there has been a sizeable gap between the two intimate acts. Other than conducting prenatal DNA testing, the only situation in which you can be sure a man is not the father is if a period of menstruation started after the last time sex had taken place with him.

In some difficult circumstances paternity results might affect a pregnant woman’s decision about whether or not to terminate the pregnancy. There are a variety of reasons why this might be, but one example arises when pregnancy has resulted from a sexual assault. There is around a 1 in 20 chance that an unplanned, randomly timed, act of intercourse will lead to pregnancy and sadly this means that each year tens of thousands of women around the world fall pregnant as a result of being victim to an act of rape. Sex crime victims are sometimes unclear whether the father of their unborn child is the attacker or their loving partner.

Types of Prenatal Paternity DNA Test
It is possible to successfully carry out prenatal paternity testing fairly early in pregnancy; some techniques have good success rates from around the end of the first trimester (10-12 weeks) but a big challenge for the technology and the physician is how to obtain a DNA sample from the fetus without harming baby or mother.  There are a variety of techniques available to obtain a sample of fetal DNA; some require an invasive procedure to collect cells by sampling fluid or tissue from within the uterus. Others are non-invasive to the uterus and rely on the fact that, during pregnancy, there is always a small number of fetal cells as well as a small amount of cell free DNA (cfDNA) from the fetus present within the mother’s bloodstream.

Invasive Sampling Procedures
There are two types of invasive fetal DNA sampling that are most likely to be used for prenatal DNA testing and, in both, samples are collected by an obstetrician or OB/GYN physician. The most frequently used is called amniocentesis. The fetus grows inside the amniotic sac and is immersed in a protective liquid called amniotic fluid. Amniotic fluid naturally contains fetal cells that can be used to test paternity of the unborn child. (Amniotic fluid can also be used to test for a number of genetic disorders.) In the procedure a small amount of amniotic fluid is drawn using a fine needle inserted through the mother’s abdomen (tummy). Ultrasound scanning, producing a live internal abdominal image, is conducted during the procedure to check the position of the fetus, find the best place to remove fluid, and guide the needle safely. Amniocentesis is typically carried out after week 14 of pregnancy and carries a slightly increased risk of miscarriage. Some clinics, in some circumstances, offer earlier amniocentesis but it is considered more risky mainly because the fetus and amniotic sac is smaller. Generally, if sampling of fetal DNA is needed earlier than 14 weeks it is conducted using a technique called Chorionic Villus Sampling (CVS), usually conducted between weeks 10 and 13. Chorionic villi are finger like protrusions in the baby’s placenta that connect with the mother’s uterus wall and across which is transferred oxygen and nutrients from mother to baby. Chorionic villus tissue, like the rest of the placenta, is derived from the fetus and contains the child’s DNA. The sampling procedure uses a thin tube that is guided through the vagina and cervix, with the aid of ultrasound scanning, to the appropriate place on the uterus wall. Sometimes, depending on the orientation of the fetus and the position of the umbilical chord, CVS might, instead, be conducted using a fine needle through the abdomen. With CVS, like amniocentesis, there is a slightly increased risk of miscarriage but it is considered safer than amniocentesis early in pregnancy.

Advantages and Disadvantages of Invasive Sampling Techniques
The main advantage of invasive sampling is that the quantity and quality of the DNA sample from the fetus is relatively good which allows STR Paternity testing to be conducted. STR testing is currently the method of choice in paternity tests because when a possible father does not match the child’s paternal STR types, he is conclusively excluded and when he matches, STRs provide powerful statistical results to reduce uncertainty often producing a probability of paternity in excess of 99.99%. Cost of STR testing using samples from amniocentesis or CVS is usually four times or more that of a standard paternity STR test and there is also likely to be a significant sample collection fee from the clinic (see The Cost of Prenatal Tests). However, it may still work out less expensive than the non-invasive approaches (as described below). The main disadvantages of invasive approaches are that there is a small but significant increased risk of miscarriage from the procedure and for this reason your doctor may advise against a decision to pursue this type of approach.

Non-Invasive Sampling Techniques
Non-invasive in this context simply means that, unlike amniocentesis or CVS, there is no attempt to sample the babies DNA directly by taking samples from inside the womb. Instead, fetal DNA is obtained from the mother’s blood stream and is collected by simply taking a sample of mother’s blood with no risk to the unborn child. Detectable Blood sample ed 2amounts of fetal DNA are present in maternal blood early in pregnancy and increases as the baby grows. There is sufficient for some types of test from about week 9 or 10. The baby’s DNA in mothers blood originates from the placenta where baby’s and mother’s tissues interlock over a large surface area.  The function of the placenta is the transfer of nutrients to the baby, waste elimination and respiratory gas exchange across mother and baby’s blood streams. The large surface area of the placenta and the close proximity of mother’s blood to baby’s tissues result in the transfer of a small amount of fetal DNA to the mother’s bloodstream. Detectable fetal DNA is of two distinct types; that which is contained within fetal cells within mothers blood and that which is uncontained and free in mother’s blood plasma (known as cell free DNA or cfDNA). The difficulties for DNA testers is are, A) that the number of fetal cells relative to the mothers cells is tiny, perhaps as few as 1 in a million and, B) whilst the proportion of fetal cfDNA compared with maternal cfDNA is much higher, up to 15%, the quality and condition of cfDNA is low because it is mostly degraded and broken in to small fragments. There are two different types of test that can be used on mother’s blood sample, as described in the paragraph below, but regardless of which type of non-invasive test is used, you can expect to pay in the region of ten times the amount of a standard test. The extra cost is because testing is more time consuming and costly for laboratories. (see The Cost of Prenatal Paternity Testing)

There are two general approaches to prenatal paternity testing using a non-invasive sample of fetal DNA from mother’s blood. One method has been practiced for the last 10 years or so and aims to detect STR types in mother’s blood that are different from mother’s known STR types and therefor must have originate from the baby’s DNA and be paternal in origin. Concentration of fetal DNA is particularly important and results are better after around 14 weeks of gestation which is equivalent to the 15th or 16th week of pregnancy. (Pregnancy is usually counted from the date of your last period but gestation starts a week or two after from the time an egg was fertilized during ovulation.) The challenge for the STR approach is that the overwhelming majority of good quality DNA in maternal blood is from the mother and it tends to swamp the small amount of good quality DNA from the fetus. This makes it hard to identify any sort of STR signal from the fetus. The challenge is slightly easier if the fetus is a male because it is possible to target STR regions that are only found on the Y chromosome and are completely absent in the mothers DNA, simplifying the interpretation of results. Regardless of whether the fetus is male or female there remain significant challenges for this approach due to the limited quantity and quality of fetal DNA. Laboratories offering this service advise that you should test all possible fathers’ to ensure all but one are excluded before nominating the true father. If it is possible to submit a sample from each possible father laboratories usually claim they achieve the correct answer around 99% of the time but offer a free standard paternity test after the baby is born as confirmation and a double check.

Pregnant Woman The second method can be carried out from around week 10. It works using a similar principle as the STR method in that it aims to identify DNA sequences in mother’s blood that are different from mother’s own DNA and so must be from fetal paternal DNA. Instead of using STRs a huge array of SNPs is deployed. As described in the home page article SNPs, or “snips”, are Single Nucleotide Polymorphisms and are points along the DNA strand where there are single nucleotide sequence variations. There are many millions of SNPs spread throughout the 3 billion nucleotides in the human genome. A great advantage of SNPs for prenatal testing is that the highly degraded nature of fetal cfDNA in mother’s blood is not a problem because only very short DNA fragments are needed to successfully give SNP results. Fetal cfDNA generally works very well. For prenatal paternity testing more than 300,000 chromosomal SNP sites can be targeted in a single test. Powerful hi-tech bioinformatics detection equipment and interpretation software is used to analyse all of these regions at once. One SNP on its own is not usually very discriminating and the test result relating to an individual SNP site may well be of no use in a given paternity test.  However, the fact that such a large number are examined means that even if only a small proportion of the 300,000 or so tests yield informative results then there will still be a significant amount of information to help determine paternity. Although the approach to statistical interpretation for SNPs is different from the standard STR approach, statistical results approximately equivalent to probabilities of paternity greater than 99.9% can often be achieved.

What If Mother-to-be is Carrying Twins or Triplets?
Invasive sampling of multiples is possible via CVS and amniocentesis but, depending on the number and arrangement of amniotic sac(s) and placenta(ae), it can be more difficult to perform and may bring additional risk of miscarriage. If you use this type of sampling, fetuses can be tested separately. As always, it is important to discuss the risks relevant to your own pregnancy with a health professional before making a prenatal sampling decision.

For non-invasive sampling: In theory, if the twins, triplets or other multiples are identical, (i.e. they originate from the same single fertilized egg – described as monozygotic) there should be no problem because each will have identical DNA and there should be no difficulties in the interpretation of the paternity test results. However, most companies offering the non-invasive SNP test advise against it because the technique has only been validated for singleton pregnancies.

If the fetuses were non-identical, fraternal multiples (di-, tri- or polyzygotic) SNP testing would certainly not be possible as there are essentially three separate types of DNA in mother’s blood which will distort the bioinformatics software algorithm used to interpret results.