Why Contraception is Necessary Even During Pregnancy

Q: What is the probability of superfetation occurring?

In medical literature, the number of reported cases worldwide is so extremely low that an accurate probability cannot be calculated. There are only a few dozen confirmed cases in humans. While relatively known in animals, it is medically very rare in humans.

Q: If superfetation occurs, can the fetuses have different fathers?

Yes, it is possible. Because the first and second pregnancies occur at different times in superfetation, if sexual intercourse with different partners happened during that period, it is possible for the two fetuses to have different fathers.

Q: What is the difference between a "0%" and "99.99%" Probability of Paternity in DNA testing?

In DNA testing, a "Probability of Paternity of 99.99%" means it is practically impossible to deny the biological relationship between the tested individuals. On the other hand, a "Probability of Paternity of 0%" indicates a denial of the biological relationship, but it includes the risk of falsely denying an actual parent-child relationship due to factors like superfetation, chimerism, mutations, or sample mix-ups. Therefore, an affirmative result is considered more reliable than a negative one.

Q: What should I do if I get a negative result (0%) in a DNA test?

If you are concerned about a negative result, you can eliminate the risk of a false negative due to misjudgment by performing an additional test with a partner likely to yield an affirmative "99.99%" result. seeDNA has a record of zero misjudgments since starting the service in 2016, and offers free consultations with specialized staff.

2025.03.28

1A Woman Who Got Pregnant While Already Pregnant
2Reliability of DNA Testing Results
3Free Consultation with Genetic Testing and DNA Experts

A Woman Who Got Pregnant While Already Pregnant

Generally, many people believe that once pregnancy is confirmed, contraception is no longer necessary.
This is likely because they feel safe assuming that since they are already pregnant, there is no risk of becoming pregnant again. However, last week in the US, the MailOnline reported on a woman who became pregnant with a second child while already pregnant.
According to the article, Taylor Henderson (28) from Texas, USA, experienced "Superfetation," an extremely rare phenomenon worldwide where a woman becomes pregnant again while already pregnant, also known as heterochronic or concurrent pregnancy.

Superfetation is a phenomenon where a pregnant woman ovulates again, and a new fertilized egg implants after sexual intercourse, resulting in the concurrent pregnancy of fetuses at different gestational ages.

According to her attending physician, Dr. Michael Rose, initial ultrasound scans showed a clear size difference between the two fetuses, leading them to initially suspect that one fetus might be experiencing delayed growth. However, as examinations progressed, it became evident that the two fetuses were actually at different gestational ages. This means that for Taylor, just two weeks after her initial pregnancy was confirmed, she ovulated again, and a different fertilized egg implanted.

Reliability of DNA Testing Results

Normally, when a woman becomes pregnant, hormones suppress ovulation, making another pregnancy impossible. Because of this, superfetation is medically very rare, with only a few cases reported worldwide.
This means that if superfetation occurs and a woman engages in sexual activity with multiple men, it could lead to a situation where the fetuses in her womb have different fathers.

This phenomenon is one of the reasons why, in fetal DNA testing performed via blood test during pregnancy, a "Probability of Paternity of 99.99%" (affirming the biological relationship between fetus and father) is more reliable than a "Probability of Paternity of 0%" (denying the biological relationship).
Generally, 0% means there is absolutely no possibility, while 99.99% implies a 0.01% chance of variation. However, in DNA testing, the implications of these numbers differ slightly. A "Probability of Paternity of 0%" in DNA testing includes risks where the biological relationship is falsely denied due to phenomena like superfetation (as in this article), chimerism, mutations, or human errors like sample mix-ups, even if there is an actual biological relationship.
A "Probability of Paternity of 99.99%" in DNA testing means "it is practically impossible to deny the biological relationship between the subjects," which provides a reassuring result.

Although the accuracy of DNA testing has improved remarkably compared to 20 years ago, perfect testing remains theoretically and practically impossible.
If you receive a "Probability of Paternity of 0%" (denial of biological relationship) and feel anxious, you can eliminate the risk of a "Probability of Paternity of 0%" due to misjudgment by testing with a partner who is likely to yield an "affirmative" result of "Probability of Paternity of 99.99%".

Frequently Asked Questions

Q.What is Superfetation (Concurrent Pregnancy)?

Superfetation is a phenomenon where a woman who is already pregnant ovulates again, and a new fertilized egg implants, resulting in the concurrent pregnancy of fetuses at different gestational ages. Normally, hormones suppress ovulation during pregnancy, but very rarely this suppression fails. In 2025, a case was reported in Texas, USA, where a 28-year-old woman became pregnant with a second child just two weeks after her initial pregnancy.

Q.What is the probability of superfetation occurring?

In medical literature, the number of reported cases worldwide is so extremely low that an accurate probability cannot be calculated. There are only a few dozen confirmed cases in humans. While relatively known in animals (like rabbits and mink), it is medically very rare in humans.

Q.If superfetation occurs, can the fetuses have different fathers?

Yes, it is possible. Because the first and second pregnancies occur at different times in superfetation, if sexual intercourse with different partners happened during that period, it is possible for the two fetuses to have different fathers.

Q.What is the difference between a "0%" and "99.99%" Probability of Paternity in DNA testing?

In DNA testing, a "Probability of Paternity of 99.99%" means it is practically impossible to deny the biological relationship between the tested individuals. On the other hand, a "Probability of Paternity of 0%" indicates a denial of the biological relationship, but it includes the risk of falsely denying an actual parent-child relationship due to factors like superfetation, chimerism, mutations, or sample mix-ups. Therefore, an affirmative result is considered more reliable than a negative one.

Q.What should I do if I get a negative result (0%) in a DNA test?

If you are concerned about a negative result, you can eliminate the risk of a false negative due to misjudgment by performing an additional test with a partner likely to yield an affirmative "99.99%" result. seeDNA has a record of zero misjudgments since starting the service in 2016, and offers free consultations with specialized staff.

References

Free Consultation with Genetic Testing and DNA Experts

In 2016, seeDNA developed Japan's first "Prenatal Fetal DNA Test" and is a pioneer in genetic and DNA testing in Japan. We are a trusted DNA testing specialist institution with "0 cases" of misjudgment since the start of our service.
If you have concerns about test results from other companies, questions about biological parent-child relationships, or worries about a partner's infidelity, our DNA testing experts will provide solid support to ensure your peace of mind. Please feel free to contact us.

Find out who the father of your baby is

Prenatal Fetal DNA Testing
Click here for details