This happens when another egg is released from the fallopian tubes and is then fertilised by another sperm. It then implants in the womb several days after the first embryo. Superfetation 'twins' are extremely uncommon. It has been known to happen in around 10 cases (opens in new tab) and most of these cases have involved some form of fertility treatment (opens in new tab).
Dr Gareth Nye, (opens in new tab) an expert in Fetal and Maternal Health at the University of Chester, told us, "For superfetation to happen successfully, a sequence of very unlikely events need to take place. Firstly, the release of an egg by an ovary during an ongoing pregnancy. This is incredibly unlikely because hormones released during pregnancy usually prevent further ovulation. The second egg is then fertilised. Finally, the second fertilised egg needs to implant in an already pregnant womb. This is difficult, as implantation requires certain hormones that usually aren't released during pregnancy."
There are some rare cases where superfetation must have occurred. One study (opens in new tab) described a case where "twins" had different fathers. The mother had sex with two different men at different times. She then experienced dual ovulation (superfetation). Paternity tests revealed that these babies have different fathers.
Research shows many cattle and pets like cats and dogs ovulate several times. These animals can have several babies with different fathers. This phenomenon is known as heteropaternal superfetation.
What happens in superfetation?
Superfetation is an unusual event that results in a woman being pregnant with two babies of different ages. It's when two eggs (ovum) are released at different times. Sometimes it can happen several days apart. The eggs are then fertilised by different sperm. This is incredibly rare, as usually, the release of hormones supporting an existing pregnancy prevents superfecundation.
Superfetation in humans almost always occurs in combination with fertility treatment. One study (opens in new tab) found superfetation occurred after intrauterine insemination (IUI). IUI involves implanting an embryo into the womb. Another study (opens in new tab) discovered a case where fertility drugs created a triplet pregnancy. Pregnant with twins, the patient got pregnant again with an ectopic pregnancy. Sadly, ectopic pregnancies (that embed outside the womb) cannot survive and usually result in miscarriage.
Dr Gareth Nye, expert in Fetal Medicine, agrees fertility treatments make superfetation likely. He explains: "Almost all recorded cases of superfetation involve IVF. A number of normal physiological processes have been skipped during IVF. This means the body does not undergo as many of the usual hormonal processes [that usually prevent superfetation]."
Although studies (opens in new tab) have demonstrated superfecundation happens in some non-human animals, it is incredibly rare in humans. The European Journal of Obstetrics & Gynecology (opens in new tab) concluded in one report that less than 10 historic cases had been recorded by the end of 2008.
However, a couple of cases worldwide have been reported since then. One case of superfetation, shared by The Independent (opens in new tab) in 2020, involved two babies, three weeks apart. The second baby was not discovered until an ultrasound (opens in new tab) scan. British mother, Rebecca Roberts, was taking fertility drugs when her babies were conceived. This lovely viral photo of her daughter and son reveals a real difference in their size. Rebecca's "super twins" raised global awareness of this incredibly rare phenomenon.
How far apart can superfetation twins be?
In theory, superfetation could occur at any time. However, all documented cases occurred within a month of the first conception. Many of the hormonal processes involved in supporting a healthy pregnancy will prevent the release of further eggs.
Fetal health expert, Dr Nye, explains the mucus plug prevents superfecundation. "During pregnancy, your cervix forms a mucus plug that blocks sperm. This mucus plug is prompted to form by pregnancy hormones."
Are there any symptoms of superfetation?
The release of two eggs, fertilised at different times, is extremely rare. There are no symptoms of superfetation. When you are already pregnant, you are unlikely to notice new signs and symptoms of pregnancy. (opens in new tab)
Usually, women are unable to detect ovulation, although some suffer from ovulation (opens in new tab) pain, known as mittelschmertz. However, it is common to have some cramping in early pregnancy. (opens in new tab) This is more likely than secondary ovulation.
Dr Gareth Nye agrees that it's unlikely that you would know if superfetation has happened. He told us "We don't have any listed symptoms, as it's so rare. The first indication would be when a doctor notices during a scan".
Can superfetation twins be prevented?
Usually, usual hormonal changes that your body undergoes during pregnancy prevent superfetation. If you are not taking fertility drugs, it is incredibly unlikely that you will experience superfetation.
Even if you are taking fertility drugs, research (opens in new tab) indicates that superfetation in humans has only occurred around 10 times.
"Your body already makes good attempts at preventing superfetation," explained Dr Gareth Nye, University of Chester's expert in Fetal and Maternal health. This is why it's so extremely rare, so it's best not to worry about getting pregnant when you are already pregnant.
Risks and complications of superfetation twins
As superfetation will result in two babies of different ages, there are risks. Twin births are considered high-risk as mums of twins are more likely to go into premature labour.
A study (opens in new tab) of 2,170 twins found that 62% of twins were born early (before 37 weeks of pregnancy). This study also found that twins are more likely to have a low birth weight.
Superfetation "twins" of different ages are more likely to need special care. Babies born too early may suffer with breathing difficulties.
Doctors usually prescribe steroids to mums who go into early labour. This helps a premature baby breathe after birth. Studies (opens in new tab) show that prescribing steroids helps a baby's lungs develop more quickly.
As it's very rare, there is no research on babies born after superfetation. Dr Nye, an expert in fetal growth, explains the risks are the same as for all premature babies.
"The youngest baby is born at the same time as the older baby. This means the [more premature] baby has a higher risk. It's common for premature babies to suffer from breathing disorders, due to immature lungs."
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