Twins with a twist

Not all siblings born on the same day are twins

FALLING PREGNANT WITH a second baby while you’re still pregnant with the first? Sounds impossible, or like the tacky storyline of a terrible soap opera, but it can, and does, happen.

TWO TIMES, A BABY…OR TWO

Falling pregnant again while already pregnant is an extremely rare and unusual phenomenon, and depending on when the second egg was released, is known as superfoetation or superfecundity – where ‘fecund’ means highly fertile or capable of producing an abundance of offspring. Superfecundity occurs when two eggs are released in one cycle and are fertilised five to seven days apart, after having sex at least twice during that time. These babies will develop like twins in two separate amniotic sacs, but can be measurably different sizes. Superfoetation, however, is much rarer – in fact the jury is still out as to whether it can happen at all – but is when one egg is released and fertilised during one cycle, and then another egg is released and fertilised in a subsequent cycle. In both cases the babies will develop and grow according to their gestational age and are often mistaken for twins, where one is just much smaller than the other. True twins are conceived by either one fertilised egg splitting in two (identical twins), or when two eggs are released and fertilised within five days of sex (fraternal or non-identical twins). Male sperm can survive in the female body for up to five days, so superfecundity refers to cases where the second egg is fertilised outside of that five-day window period.

IN THE NORMAL COURSE OF EVENTS

Usually when a woman falls pregnant, a flurry of pregnancy hormones kicks in, preventing the ovaries from releasing any more eggs. Even if an egg is released, then this same set of hormones causes changes in the uterus so that any other sperm should not be able to get through, as well as causing changes to the lining of the uterus preventing another fertilised egg from being able to attach itself to the womb. In superfoetation, however, those pregnancy hormones are off-kilter and don’t do the job properly, so that another egg is released the following month. If this second egg is fertilised then this baby will be a month or more younger than his or her sibling.

THE RUNT OF THE LITTER

Although superfoetation and superfecundity is common in many animal species, such as cats, dogs and rabbits, “we used to think that superfecundity and superfoetation didn’t exist in humans, but there are a few reported cases,” says Cape Town-based OB/GYN, Dr Martin Puzey. In animals, the female goes on ‘heat’, releasing eggs, for between two and three weeks in dogs, and can mate with many males.