Known as superfetation, the (extremely) unlikely occurrence of falling pregnant when already pregnant happens when more than one egg is released during the menstrual cycle, and both become fertilised in the uterus.
In the most recent and shocking case reported just last year, an Australian couple even claimed they only had sex once, creating two separate embryos 10 days apart.
Kate Hill, a sufferer of PCOS, was struggling to conceive with husband Peter and had been undergoing hormone therapy to stimulate ovulation. Yet neither could have predicted the success of the treatment when two eggs were fertilised, with a gestational age of roughly 10 days apart.
The couple maintain that Peter's sperm stayed alive unfertilised in the womb for over a week, before creating a second embryo – who became Charlotte, born 10 days after her older 'twin' Olivia.
Experts are keen to note that biologically, siblings born through superfetation are not actually twins. Created by two different ovums and two different sperms.
Speaking to Broadly, gynaecologist and author of She-ology Dr. Sherry A. Ross said: 'A woman ovulates, she gets pregnant, she creates an embryo, and then a few days or even a week later she ovulates again, and then gets pregnant again, creating a second embryo.'
This means the process of superfetation is categorically different to the conception of twins. And though the natural conception of twins is incredibly rare, superfetation continues to baffle doctors – despite a small handful of cases being reported in recent history.
'Doctors especially like hard facts; they like to show a trail as to how this actually makes sense, and I think that's sort of what makes this a mystical entity for the medical community,' adds Dr Sherry.
In 2009, a similar case was reported when Todd and Julia Gorvenburg, from Arkansas, learnt a younger male foetus was growing beside his older sister in the womb, who had been conceived a few weeks before. Both babies were born on the same day, but were not classified as twins due to their differing gestational ages.
Normally, of course, there are multiple mechanisms which stop the female body falling pregnant when it's already hosting an embryo.
'If you were to get pregnant during your ovulation, then your ovaries shut down and they should not ovulate anymore,' says Dr Sherry. 'You get pregnant, your eggs [are] fertilized, and then your ovaries stop producing future follicles.'
The body also creates a 'mucus plug which forms in the cervix after conception, and prevents sperm from entering the uterus for that second egg to get fertilized.'
So there's no need to swear off future pregnancies just yet - but it is worth noting that couples undergoing IVF are (minimally) more likely to experience the phenomenon. '[If] you're getting fertility treatment, you're defying what typically happens in your body,' she points out.
Superfetation, when it does occur, doesn't generally carry any more risks than a normal pregnancy.