Twins 101: The ULTIMATE Guide To Understanding Twins

Sep 3

So you just found out you’re pregnant with twins — or maybe you’re just twin-curious. Either way, welcome to double the questions, double the scans, and double the diapers.

Let’s break down the types of twins, the risks, and what makes twin pregnancies so unique.

The Basics of Twins

Fraternal Twins (Dizygotic)

Two eggs + two sperm = two completely separate genetic blueprints.
Basically like siblings… born at the same time.
About 70% of twin pregnancies are fraternal.

Fun fact: If you ovulate more than one egg and happen to have intercourse with more than one partner during your fertile window, fraternal twins could technically have different dads. Yep, daytime talk show producers everywhere are salivating.

Identical Twins (Monozygotic)

One egg + one sperm that split into two embryos.
They share identical genetics (same sex, same DNA, sometimes mirror-image features).
About 30% of twin pregnancies are identical.

Timing = Twin Type

When that fertilized egg decides to split determines what type of twins you’ll have:

Days 1–3: Di-Di (Dichorionic-Diamniotic)

Two placentas, two amniotic sacs.
Each baby has its own food source and protective bubble.
Safest and most common setup for identicals.

Days 4–8: Mo-Di (Monochorionic-Diamniotic)

One placenta, two amniotic sacs.
Babies share nutrients but have their own sacs.
Risk: Twin-to-Twin Transfusion Syndrome (TTTS).

Days 8–13: Mo-Mo (Monochorionic-Monoamniotic)

One placenta, one sac.
The high-risk “roommates” — they share everything, including the risk of cord entanglement.
Survival rate ~70%.

Day 13+: Conjoined Twins

Rare, complex, and always high-risk.
Twins remain physically fused.
Survival rate ~7.5%.

Risks to Know

Twin-to-Twin Transfusion Syndrome (TTTS)

Happens in Mo-Di or Mo-Mo pregnancies when blood flow is unevenly shared.
One baby (the “donor twin”) gives more than it receives, often leading to growth restriction and low amniotic fluid.
The other baby (the “recipient twin”) receives too much, leading to heart strain and excess fluid.
Affects 10–15% of monochorionic pregnancies.

Mo-Mo Complications

Cord entanglement (babies floating in the same sac, so their umbilical cords can wrap around each other).
Higher chance of miscarriage, stillbirth, or cerebral palsy if oxygen is compromised.

Conjoined Twins

Occurs when splitting happens after 13 days and embryos remain fused.
Can be joined at the chest, abdomen, pelvis, or — very rarely — the cranium.
Some can be separated surgically depending on which organs are shared, but many cannot.

Why This Matters

Twin pregnancies are considered high-risk because of these potential complications, and they require more frequent monitoring, ultrasounds, and specialized care. But don’t let that scare you — twin parents will also tell you it’s double the love, double the snuggles, and yes… double the chaos.

If you’re expecting twins, your care team will walk you through the specifics of your pregnancy type and help prepare you for birth. Knowledge is power, and the more you understand, the more confident you’ll feel.