How Twins, Triplets and More Are Made: The Full Inside-the-Body Story
The two recipes
Fraternal (dizygotic) twins happen when two eggs are released in one cycle — hyperovulation — and both are fertilised. It runs in families through the mother's line (a gene tendency to release two eggs), becomes more likely with maternal age up to the late 30s, and is more common with fertility treatment. Each baby has its own placenta and sac. Identical (monozygotic) twins begin as one egg, one sperm — then the early embryo divides. Nobody knows exactly why; it happens at roughly the same rate everywhere on earth, in about 1 in 250 pregnancies. In this video I take you inside, day by day, to see both stories unfold.
Timing is everything with identical twins
- Split in the first 3 days: two placentas, two sacs — looks just like fraternal twins on a scan.
- Split on days 4–8: one shared placenta, two sacs (the most common identical arrangement) — needs extra monitoring because the babies share a blood supply.
- Split on days 8–13: one placenta, one sac — rare, and watched closely because the cords can tangle.
- Split after day 13: incomplete separation — conjoined twins, extremely rare.
Carrying more than one
A multiple pregnancy is automatically watched more closely: more scans (especially for shared placentas), earlier screening for anaemia and blood pressure, and a delivery plan made with your team — multiples tend to arrive early, averaging around 36–37 weeks for twins. If you've just seen two heartbeats on a scan: congratulations. Your care team will do the extra watching; your job is the same as any pregnancy — eat well, rest more than you think you need, and ask every question twice.
Common questions
Do twins skip a generation?
The old saying has a grain of truth for fraternal twins: the hyperovulation tendency passes through the mother's line, so a man can carry it silently and pass it to his daughters. It doesn't truly 'skip' — it just hides.
Can a scan tell identical from fraternal?
Often, yes: the number of placentas and the membrane between the sacs give strong clues. Twins of different sexes are essentially always fraternal. A DNA test after birth gives certainty when the scan can't.
Are triplets always from fertility treatment?
No — natural triplets occur, roughly 1 in several thousand pregnancies, though rates rose with fertility treatment. Triplets can be three separate eggs, a pair-plus-one, or one egg that split twice.
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