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What do you call twins not born on the same day?

Twins that are born on different days are called heteropaternal superfecundation twins. This phenomenon occurs when two separate eggs from the same mother are fertilized by two different fathers and implanted in the uterus at different times, leading to the babies being born on different days. While rare, heteropaternal superfecundation does occasionally happen. Let’s explore how this can occur and what implications it has.

What Causes Heteropaternal Superfecundation Twins?

For heteropaternal superfecundation twins to occur, a few things need to line up:

  • The mother releases two eggs during ovulation rather than just one. This is called superfecundation.
  • The mother has sex with two different partners within the same ovulation period of 5 days.
  • Each egg is fertilized by a different father’s sperm.
  • The fertilized eggs implant in the uterus days apart, leading to different gestation periods.

Superfecundation is rare, with only about 1 in 400 pairs of fraternal twins being heteropaternal. The mother needs to release more than one egg while ovulating, which happens occasionally but not commonly. Then, the timing has to work out so she has sex with different partners within a 5 day period to separately fertilize each egg.

When Do Heteropaternal Superfecundation Twins Occur?

For heteropaternal twins to occur, the mother needs to have sex with two partners within the same short fertility window. This likely happens in a few scenarios:

  • The mother is sexually active with multiple partners around the time of ovulation.
  • The mother is separated but has sex with both her ex-partner and new partner in the same week.
  • An assault leads to unwanted fertilization by a second man.

While less common today, in the past when fertility tracking was less understood, heteropaternal twins may have occurred more often from multiple partners within the ovulation window.

How Do Heteropaternal Superfecundation Twins Develop?

When two eggs are separately fertilized by two different fathers within the ovulation period, they will implant and develop on slightly different schedules:

  • Egg 1 is fertilized by Father 1’s sperm on Day 1 of ovulation.
  • The fertilized Egg 1 implants in the uterus shortly after and starts developing.
  • On Day 4 of the same ovulation period, Egg 2 is released and fertilized by Father 2’s sperm.
  • Fertilized Egg 2 implants a few days after Egg 1, leading to a staggered development.
  • The babies have different due dates based on their implantation days.

The staggered implantation means the twins develop at slightly different paces. They may be born up to a week or more apart, yet still be considered twins.

Gestation and Birth of Heteropaternal Twins

The twins will be monitored carefully throughout pregnancy as doctors realize they have different gestational ages. While rare, some key points about their development and birth include:

  • Each twin may have a different due date based on implantation.
  • The first twin is often born prematurely to accommodate the second’s due date.
  • A C-section delivery is common to control timing.
  • The twins may have quite different birth weights and development.

Doctors aim to find an optimal delivery time for both twins. The births may be spaced days or even weeks apart in rarer cases.

Genetic Differences Between Heteropaternal Superfecundation Twins

Given they have different biological fathers, heteropaternal twins are genetically half-siblings:

  • They share the same mother but have different fathers.
  • Their genes will vary as much as half-siblings rather than identical or fraternal twins.
  • They may look quite different at birth and as they grow older.
  • Standard twin genetic testing will uncover they only share half their genes.

In rare cases where heteropaternal twins look somewhat alike, DNA testing may be required to uncover if they have different fathers.

DNA Testing to Confirm Heteropaternal Twins

DNA testing can conclusively identify if twins have different fathers:

  • Paternity test: Compares twins’ DNA to suspected fathers.
  • Zygosity test: Compares twins’ DNA to assess relatedness.
  • Genetic markers: Identify ancestry and traits from different fathers.

Without DNA evidence, heteropaternal twins may never be identified if no paternal discrepancy is suspected. Testing gives an accurate answer in uncertain cases.

Key Implications of Heteropaternal Superfecundation

There are some important considerations for heteropaternal twins regarding health, legality, and ethical issues:

Health Implications

  • Higher odds of preterm birth and low birth weight
  • Developmental differences due to varied gestation
  • May need greater neonatal care after birth

Closer monitoring during pregnancy allows doctors to prepare for any complications.

Legal and Ethical Concerns

  • Establishing legal paternity and parental rights
  • Child support disputes if fathers unknown
  • Potential stigma around rare conception

Legal counsel may be needed to resolve disputes if fathers unaware. There is sometimes stigma around the rare double conception.

Psychological Impact on the Family

  • Stress around contested paternity
  • Emotional strain on fathers, children
  • Potentially complex family dynamics

Counseling can help address sensitivities and promote bonding between children and their fathers.

Famous Cases of Heteropaternal Superfecundation

While rare, there are a few famous cases of heteropaternal twins:

The “Telephone Twins” of England

  • Born in 1995 to married couple Lydia and Vince Fair.
  • Suspicions around different skin tone led to DNA tests.
  • Tests found twins had different biological fathers.

The father was apparently out of town when the second egg was fertilized by another man.

Twin Boys Born in Vietnam

  • Born in 2008 to mother Pham Thi Dinh.
  • Twins were born 4 days apart.
  • DNA tests confirmed different fathers.

The mother admitted to having relations with two men when the rare double conception occurred.

Mixed-Race Twins in the Netherlands

  • Born in 2002 to a Dutch mother and Moroccan father.
  • Twins born with noticeably different skin tones.
  • DNA tests showed the father was only related to the pale twin.

This case brought attention to the phenomenon rarely occurring outside fertility treatments.


Heteropaternal superfecundation twins, while extremely rare, do occasionally occur when two eggs from the same mother are fertilized by different fathers. For this to happen, the mother must release multiple eggs during ovulation and have relations with two partners in the same short fertility window. The twins are born days or even weeks apart with different due dates. DNA tests can confirm their lack of full genetic identity. These special twins bring unique considerations around legal, ethical, and psychological issues. But with openness and care for all involved, families can embrace their extraordinary origins.