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 and most of these cases have involved some form of fertility treatment.
Dr Gareth Nye, 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 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 found superfetation occurred after intrauterine insemination (IUI). IUI involves implanting an embryo into the womb. Another study 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 have demonstrated superfecundation happens in some non-human animals, it is incredibly rare in humans. The European Journal of Obstetrics & Gynecology 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 in 2020, involved two babies, three weeks apart. The second baby was not discovered until an ultrasound 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.
Usually, women are unable to detect ovulation, although some suffer from ovulation pain, known as mittelschmertz. However, it is common to have some cramping in early pregnancy. 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 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 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 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."
Related video: Old wives' tales to predict the sex of your baby
Parenting advice, hot topics, best buys and family finance tips delivered straight to your inbox.
Tannice Hemming has worked alongside her local NHS in Kent and Medway since she became a parent and is now a mum of three. As a Maternity Voices Partnership Chair, she bridged the gap between service users (birthing women and people, plus their families) and clinicians, to co-produce improvements in Maternity care. She has also worked as a breastfeeding peer supporter. After founding the Keep Kent Breastfeeding campaign, she regularly appears on KMTV, giving her views and advice on subjects as varied as vaccinations, infant feeding and current affairs affecting families. Two of her proudest achievements include Co-authoring Health Education England’s E-learning on Trauma Informed Care and the Kent and Medway Bump, Birth and Beyond maternity website.
This baby name is giving 'retro chic' vibes and it's predicted to be a top choice for parents in 2024 – would you choose it?
It’s stolen the limelight right at the last minute
By Daniella Gray Published
Rochelle Humes' adorable note left by her six-year-old shows how quickly kids pick up on big changes at home - psychologist shares her top tips on how best to prepare them
Valentina picks up on what will help her mummy while daddy is away
By Selina Maycock Published
Fertility foods: 21 of the best foods for getting pregnant, according to a nutritionist
Making sure your diet is packed full of fertility food can improve your chances of getting pregnant. Here’s some of the best foods to eat
By Emily-Ann Elliott Last updated
How early on can you take a pregnancy test and which pregnancy test can you take the earliest?
How early on can you take a pregnancy test, and how accurate are they? All of your pregnancy test questions, answered
By Dr Larisa Corda Last updated
Gender selection: how to conceive a boy or a girl
Everything you need to now about conceiving a boy or a girl...
By Tannice Hemming Published
Why can't I get pregnant? 10 possible reasons you're struggling to conceive
If you've been trying for a while but haven't had any success yet, read these 10 possible reasons you've having problems getting pregnant.
By Debra Waters Last updated
I want a baby and they don't: what to do when your partner's not ready to take the next step
Wanting a baby when your partner doesn't is a far more common problem than you might think
By Grace Walsh Published
When to get pregnant: the best time of year for a baby
Did you know the season your child is born in can affect its health, as well as making a big impact on you and your health too?
By Jessica Dady Published
What is ovulation? Everything you need to know about ovulation
Getting pregnant should be easy, but it isn't always. So, read all about ovulation here, find out when you're most fertile and try our ovulation date calculator
By Dr Larisa Corda Last updated
How to get pregnant: 8 expert tips from fertility expert Larisa Corda
Wondering how to get pregnant or are in the midst of trying? This Morning's fertility expert Dr Larisa Corda shares her top tips for boosting your chances of conceiving.
By Dr Larisa Corda Last updated