Identical and Non-Identical Twins Explained

Two babies at once happens in two quite different ways.

Identical twins come from a single fertilised egg that splits in two. Non-identical twins come from two eggs fertilised at the same time, and are no more alike than any other siblings.

Updated 16 October 2026 · 5 sources

One egg splits
Two eggs, two babies
Twins help science
Twins in three ideas

Check what changes the odds

THE SHORT ANSWER

Two kinds of twins,
two different stories.

The NHS says one third of twins are identical
and two thirds are non-identical.

IDENTICAL

One egg, split in two

Identical twins share the same genes and are always the same sex. The NHS says nobody knows what causes them, and everyone has the same chance: about 1 in 250.

Check the odds

NON-IDENTICAL

Two eggs at once

Non-identical twins come from two eggs fertilised in the same cycle. They can be the same sex or different, and they are more likely to run on the mother's side.

See the numbers

TWIN STUDIES

Nature and nurture

Comparing identical and non-identical twins lets researchers estimate how much of the variation in a trait comes from genetic differences.

How it works

Two seedlings growing in matching clay pots on a sunny windowsill, one taller than the other — AI-generated image, not a photograph of a real person or place.

HOW TO TELL TWINS APART

Identical or not?
Clues and the real test.

These come from the NHS page on twin pregnancies. Sometimes the answer is clear early on; sometimes only a test after birth can say.

Different sexes
A boy and a girl are always non-identical, because identical twins are always the same sex.
The 12-week scan
It is usually possible to find out you are having twins at the 12-week scan.
The placenta
Twins sharing a placenta are identical; two separate placentas can mean either kind.
A DNA test
The most accurate way to tell, and it can only be done after the babies are born.

TWO NUMBERS WORTH KNOWING

Fairly rare by nature,
much rarer by design.

One figure on identical twins worldwide,
one on how UK fertility clinics cut multiple births.

3–4 per 1,000

Identical twins per 1,000 births worldwide

MedlinePlus Genetics says identical twins occur in 3 to 4 per 1,000 births worldwide, and non-identical twins are about twice as common.

IDENTICAL TWINS · MEDLINEPLUS

24→10%

Multiple birth rate after IVF, 2008 to 2017

The UK fertility regulator met its 10% multiple birth target for the first time in 2017, down from 24% in 2008, mainly by transferring one embryo instead of two.

IVF MULTIPLE BIRTHS · HFEA

The HFEA calls multiple births the single greatest risk of fertility treatment, because multiple pregnancies and births are riskier for the mother and babies.

A QUICK ODDS CHECK

What changes
the chance of twins?

Pick a situation to see what NHS, MedlinePlus Genetics and HFEA information says about it. The bar shows how much it raises the chance of twins.

Based on NHS, MedlinePlus Genetics and HFEA pages. General information only, not medical advice; ask your midwife or clinic about your own pregnancy or treatment.

Twin odds check

Choose a situationIdentical twins in the familyA sister had non-identical twinsBeing pregnant over 35IVF with two embryos transferredIVF with a single embryo

Identical twins in the familyNo change

Identical twins do not run in families, the NHS says. Everyone has about the same chance, around 1 in 250, and most cases are not thought to be genetic.

A sister had non-identical twinsAbout 2× more likely

MedlinePlus Genetics says having non-identical twins is about twice as likely if a close relative, such as a sister, has had them.

Being pregnant over 35More likely

The NHS says non-identical twins are more likely over 35, because you are more likely to release more than one egg in a cycle.

IVF with two embryos transferredMuch more likely

When the HFEA began its campaign in 2007, one in four IVF births were multiple births, 20 times higher than natural conception.

IVF with a single embryoMuch lower

The HFEA says elective single embryo transfer, in the right patients, gives a similar pregnancy rate to transferring two, with a much lower multiple birth rate.

Sources: NHS, Pregnant with twins; MedlinePlus Genetics, Is the probability of having twins determined by genetics?; HFEA, Our campaign to reduce multiple births.

Two identical green pears side by side on a wooden table, with a red apple a little apart — AI-generated image, not a photograph of a real person or place.

WHY SCIENTISTS LOVE TWINS

Same genes,
different lives.

Identical twins have almost no differences in their DNA, while non-identical twins share about half on average. If a trait is more alike in identical pairs raised together, genes probably play a part. Comparing the two gives an estimate of heritability.

Heritability describes variation in a population, not how much of one person's trait is caused by genes. TwinsUK, founded in 1992 at King's College London, now includes over 16,000 identical and non-identical twins aged 18 to 104.

In conversation

Questions people ask

What is the difference between identical and non-identical twins?

Identical twins come from one fertilised egg that divides in two and share the same genes. Non-identical twins come from two eggs fertilised at the same time and are no more alike than other siblings.

Do twins run in families?

Non-identical twins can, usually on the mother's side, probably through an inherited tendency to release more than one egg. Identical twins do not run in families, according to the NHS.

Which kind of twins is more common?

Non-identical twins. The NHS says one third of all twins are identical and two thirds non-identical, and MedlinePlus says non-identical twins are about twice as common.

Can identical twins be a boy and a girl?

No. The NHS says identical twins are always the same sex, so a boy and a girl are non-identical twins.

Does a heritability of 70% mean a trait is 70% genetic?

No. MedlinePlus Genetics explains that it means 70% of the variability in the trait in a population is due to genetic differences, not that 70% of it is caused by genes in any one person.

Sources

Each figure on this page comes from the sources below.

One egg or two,
that is the question.