# Chorionicity and Zygosity in Twins

> Chorionicity and zygosity in twins for NEET-PG Obstetrics and Gynaecology, covering the T and lambda signs, timing of splitting and prognostic implications.

- Canonical URL: https://prepelephant.com/topics/neet-pg/obstetrics-and-gynaecology/chorionicity-and-zygosity-in-twins
- Exam / course: NEET-PG · Subject: Obstetrics and Gynaecology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-08
- Last updated: 2026-10-08
- Reference basis: Williams Obstetrics; Dutta's Textbook of Obstetrics; ISUOG Practice Guidelines on Twin Pregnancy
- How to cite: "Chorionicity and Zygosity in Twins", PrepElephant, https://prepelephant.com/topics/neet-pg/obstetrics-and-gynaecology/chorionicity-and-zygosity-in-twins

## Direct answer

Chorionicity describes the number of chorionic sacs and placentas in a twin pregnancy, and it is determined mainly in the first trimester by ultrasound. The lambda (twin peak) sign indicates dichorionic twins, whereas the T sign indicates monochorionic diamniotic twins, and both are read best in the first trimester. Monochorionic twins carry higher perinatal risk. Zygosity, whether twins are identical or non-identical, cannot be determined by ultrasound alone.

## What you must remember

- **Dichorionic diamniotic:** Two chorions, with a thick membrane whose base shows the lambda (twin peak) sign.
- **Lambda sign:** A triangular projection of chorionic tissue into the base of the intertwin membrane, indicating dichorionicity; it is most reliable in the first trimester.
- **T sign:** The thin intertwin membrane meets the placental surface at a right angle with no chorionic tissue between the layers, indicating monochorionic diamniotic twins.
- **Splitting timing:** Splitting on days 0 to 3 gives dichorionic diamniotic twins, days 4 to 8 monochorionic diamniotic, days 8 to 12 monochorionic monoamniotic, and after day 13 conjoined twins.
- **Zygosity:** Opposite-sex twins are always dizygotic, and same-sex twins may be either identical or non-identical.
- **Monochorionic risks:** Twin-to-twin transfusion syndrome, twin anaemia polycythaemia sequence, selective growth restriction and twin reversed arterial perfusion.
- **Monochorionic monoamniotic risk:** The highest perinatal risk, mainly from cord entanglement.

## Chorionicity maps onto the timing of splitting

Chorionicity is set by when the embryo splits, which is why it maps onto membrane appearance. The table links each type with its splitting window, membrane sign and main risk.

| Type | Splitting window | Membrane sign | Key risk |
|---|---|---|---|
| Dichorionic diamniotic | Days 0 to 3 | Lambda sign, thick membrane | Lowest risk |
| Monochorionic diamniotic | Days 4 to 8 | T sign, thin membrane | TTTS and TAPS |
| Monochorionic monoamniotic | Days 8 to 12 | No separating membrane | Cord entanglement |

Zygosity is a separate question. Monochorionic twins are always identical, but dichorionic twins may be either identical or non-identical, so zygosity needs DNA testing after birth when it matters clinically.

Placental examination after birth can suggest chorionicity, but it is not a substitute for early ultrasound. Counselling should be honest about what the scan can and cannot show, especially for zygosity. Monochorionic pregnancies also need closer surveillance for growth and fluid discordance from mid-pregnancy.

Chorionicity is recorded on the first scan and confirmed at the anomaly scan, so the surveillance plan can be set early.

## Where the exam tries to catch you

Questions test the membrane signs, the timing window and the link between monochorionicity and zygosity.

- **Trap:** Assuming same-sex twins are monozygotic. — **Correct:** Same-sex twins may be dizygotic, so zygosity needs genetic testing after birth rather than ultrasound.
- **Trap:** Swapping the two signs, so that the T sign is called dichorionic. — **Correct:** The lambda (twin peak) sign with a thick membrane means dichorionic, and the T sign with a thin membrane means monochorionic diamniotic.
- **Trap:** Leaving chorionicity until the second half of pregnancy. — **Correct:** Both signs are best assessed in the first trimester, and the lambda sign becomes harder to see as pregnancy advances.
- **Trap:** Assuming dichorionic twins have no risks. — **Correct:** Dichorionic twins still face growth discordance and preterm birth, though twin-to-twin transfusion syndrome does not occur.

## Frequently asked questions

### What is the T sign in twin pregnancy?

The T sign is an ultrasound finding in which the thin intertwin membrane meets the placenta at a right angle, with no chorionic tissue between its layers. It indicates monochorionic diamniotic twins. Dichorionic twins instead show the lambda sign at the membrane base.

### What is the lambda sign and when is it seen?

The lambda sign is a triangular projection of chorionic tissue at the point where the thick membrane inserts into the placenta, and it indicates dichorionic twins. It is most reliable in the first trimester and becomes harder to see later. Its absence with a T sign points to monochorionic twins.

### Can ultrasound determine whether twins are identical?

Ultrasound cannot reliably determine zygosity in most cases. Monochorionic twins are always identical, but dichorionic twins can be either identical or non-identical. Zygosity is confirmed by DNA testing after birth when clinically needed.

### How does the timing of splitting determine chorionicity?

Splitting within the first three days after fertilisation gives dichorionic diamniotic twins, while splitting from day 4 to 8 gives monochorionic diamniotic twins. Later splitting produces monoamniotic twins, and splitting after about day 13 produces conjoined twins.