# Di/Di, Mo/Di and Mo/Mo Twins: What Chorionicity Means for Your Pregnancy

_Three letters on your scan report decide how often you are scanned and what week your team aims for. Here is how to read them._

- Source: https://proba.baby/en/blog/chorionicity-di-di-mo-di-mo-mo-twins
- Published: 2026-09-05
- Author: ProbaBaby Team
- Category: Twins & Multiples
- Tags: twins, multiples, tests, first-trimester
- Language: English

> Di/di twins have two placentas and two sacs, mo/di share a placenta, mo/mo share both. Here is how the 11-14 week scan tells them apart with the lambda and T sign, what changes in your scan schedule, and how each type shifts the week your babies are likely to arrive.


> **Medical disclaimer.** This article explains terminology you will see on a twin scan report. It is general information, not medical advice, and it cannot tell you what type of twin pregnancy you have — only your scan can do that. Monitoring schedules and birth timing are set by your own maternity team, and national guidance differs between countries. If you have bleeding, pain, reduced movements or any concern about your babies, contact your midwife or maternity unit straight away.

If you are trying to work out **di/di vs mo/di twins**, you are probably holding a scan report with letters on it that nobody explained. Here is the short answer. Chorionicity is how many placentas your babies have, and amnionicity is how many sacs. **Di/di (DCDA)** twins have a placenta each and a sac each. **Mo/di (MCDA)** twins share one placenta and have separate sacs. **Mo/mo (MCMA)** twins share both. That single line is the most useful thing on the whole report, because it decides how often you are scanned and what week your team will aim for.

It is not a grade, and it is not something you did or could have changed. It is a piece of anatomy settled in the first fortnight after conception, and knowing it early is good news — it is what lets your team give you the right schedule instead of a generic one.

## What di/di, mo/di and mo/mo actually mean

Two questions, asked in order.

- **How many placentas?** Two placentas is *dichorionic*. One shared placenta is *monochorionic*.
- **How many sacs?** Two sacs is *diamniotic*. One shared sac is *monoamniotic*.

Stack the answers and you get the three labels:

- **Di/di — dichorionic diamniotic (DCDA).** Two placentas, two sacs. The most common arrangement, and the one with the fewest twin-specific complications, because there is no shared circulation between the babies.
- **Mo/di — monochorionic diamniotic (MCDA).** One placenta, two sacs. StatPearls puts this at about **1 in 5 twin pregnancies**. The babies are separated by a thin membrane but share a placenta, and therefore share blood vessels.
- **Mo/mo — monochorionic monoamniotic (MCMA).** One placenta, one sac, no dividing membrane. Genuinely rare — StatPearls gives an incidence of about **8 per 100,000 pregnancies** — and always cared for at a specialist centre.

## How the 11–14 week scan tells them apart

Where the dividing membrane meets the placenta, the sonographer looks at the shape of the join.

- In a **di/di** pregnancy there are two placentas. Where they lie side by side and fuse, a thick wedge of placental tissue pushes up into the base of the membrane, and on screen that wedge looks like a triangle: the **lambda sign**, also called the twin peak sign. Two placentas sitting clearly apart answer the same question a different way — the report counts two placental masses.
- In a **mo/di** pregnancy there is only one placenta, so the thin membrane meets it at a right angle with no wedge at all. That is the **T sign**.
- In a **mo/mo** pregnancy there is no dividing membrane to look at.

The ISUOG practice guidelines say chorionicity "should be determined prior to 13 + 6 weeks of gestation based on as many ultrasound characteristics as possible, including the entire intertwin septum (at the site of insertion of the amniotic membrane into the placenta, using the lambda sign or the T sign), membrane thickness and number of placental masses." The NHS says the same thing in fewer words: if your first ultrasound scan is done before 14 weeks, it should be possible to tell accurately what kind of placenta your twins have.

The deadline is real. As the pregnancy goes on, the membrane thins and the lambda wedge gradually flattens out, so the sign that was obvious at 12 weeks can be ambiguous at 20. This is why the [11–14 week dating scan](/medical-tests) carries more weight in a twin pregnancy than in a singleton one — it is doing a job that gets harder every week it is delayed.

## Does this tell me whether my twins are identical?

Only in one direction, and this trips up almost everybody.

The NHS states it cleanly: at the scan you should be told whether the babies share a placenta, "meaning they're identical", or whether they have two separate placentas, "meaning they can be identical or not."

So **mo/di and mo/mo answer the question** — a shared placenta means one egg split, so the babies are identical. **Di/di does not.** Two placentas is the normal arrangement for fraternal twins, but an egg that splits early enough also produces two placentas. Plenty of di/di twins turn out to be identical. If you want certainty with a di/di pair and the babies are the same sex, the answer comes from a DNA zygosity test after birth, not from an ultrasound.

## What actually changes if the babies share a placenta

A shared placenta means shared blood vessels running between the two babies. Usually the flow across them is balanced. When it is not, one twin can end up transfusing the other — twin-to-twin transfusion syndrome — and related problems can affect growth. StatPearls estimates these vascular complications affect **around 15% of mo/di twin pregnancies**.

That number is the reason for the whole difference in care. These conditions develop gradually and are picked up on ultrasound, so the response is simply: scan more often, and scan earlier.

ISUOG describes the pattern for uncomplicated pregnancies: monochorionic twins get a first-trimester scan and then **one every two weeks from 16 weeks onwards**, while dichorionic twins get the first-trimester scan, the anomaly scan at around 20 weeks, and **one every four weeks after that**. UK unit guidance built on NICE NG137 sets the same two speeds from its own start points — fortnightly fetal ultrasound assessment from 16+0 weeks in uncomplicated monochorionic twins, and growth scans every four weeks from 24 weeks for di/di. All monoamniotic pregnancies are referred to a tertiary centre with expertise in managing them.

So from about halfway, a mo/di pregnancy usually means being at the hospital twice as often as a di/di one. Your team sets your actual schedule; these are the shapes it is built from.

## What changes about your due date

Your medical due date — the 40-week estimate, [worked out the usual way](/blog/how-due-date-calculators-work) — does not change, and [why a twin pregnancy carries two dates](/blog/twin-due-date-explained) is a topic of its own. What changes here is the week your team aims for, and the honest position is that guidance differs between countries.

| Type | UK-side planned birth | US-side planned birth |
|---|---|---|
| Di/di (DCDA) | 37+0 – 37+6 | 38+0 – 38+6 |
| Mo/di (MCDA) | 36+0 – 36+6 | 34+0 – 37+6 |
| Mo/mo (MCMA) | 32+0 – 33+6 | 32+0 – 34+0 |

The UK column follows NHS unit guidance aligned to NICE NG137; the US column is from StatPearls. ISUOG, writing internationally, gives 37+0 to 37+6 for uncomplicated dichorionic twins and 36+0 to 36+6 for uncomplicated monochorionic twins. To see those windows against your own dates, the [twin due date calculator](/tools/twin-due-date-calculator) shows the 40-week medical due date and the chorionicity-adjusted target and window together. The direction is consistent even where the numbers are not: **the more the babies share, the earlier the planned birth.**

Real-world data lands in the same place: in a 2023 twin cohort, monochorionic pregnancies were delivered at a median of 36 weeks against 37 weeks for dichorionic ones.

Two things follow from that table. First, a 40-week countdown is the wrong shape for a twin pregnancy — see [twin pregnancy tracking](/twins) for what a twin-adjusted timeline looks like. Second, "earlier" is not a warning. A planned birth at 36 weeks is a plan, not an emergency, and it exists precisely because the alternative was studied.

## When the report says it could not be determined

Sometimes the scan is late, the position is awkward, or the membrane is genuinely hard to read. Guidance for that case is deliberately cautious, and it is worth knowing so the wording does not frighten you.

ISUOG advises a second opinion from a tertiary referral centre, and adds: "If the center is uncertain about the chorionicity, it is safest to classify the pregnancy as monochorionic." UK unit guidance says the same — manage as monochorionic until proved otherwise.

In other words, uncertainty resolves toward *more* care, never less. If your notes say "chorionicity uncertain — managed as monochorionic", you are being given the fortnightly schedule as a precaution.

## How to find it on your own report

You are looking for one of these, usually near the top of the ultrasound report:

- The abbreviations **DCDA**, **MCDA** or **MCMA** — sometimes written DC/DA, MC/DA, MC/MA.
- The words **dichorionic**, **monochorionic**, **diamniotic** or **monoamniotic**.
- A line about the **lambda sign**, **T sign**, **twin peak sign**, or **number of placental masses**.

If none of that is there, it is a perfectly ordinary question for your next appointment, and a good one to write down in advance — your [appointment notes](/doctor-visit) are the right place for it. Chorionicity is a fact about your pregnancy that shapes your whole schedule, and your team will expect you to ask.

## How ProbaBaby helps

If you have told ProbaBaby you are expecting twins, the pregnancy profile asks one question — whether your scans say the babies share a placenta — with **DCDA, MCDA, MCMA and "Not sure yet"** as the answers, phrased as something you copy off the 11–14 week scan report rather than a medical question to answer yourself. The answer moves the twin delivery target the app shows: **37+0 for DCDA or unknown (window 34+0–38+6), 36+0 for MCDA (34+0–37+6), 33+0 for MCMA (32+0–33+6)**, grounded in NICE NG137 and ACOG Practice Bulletin No. 231 — with your medical 40-week due date kept alongside it, never overwritten. If your profile says the babies share a placenta, the Medical Tests section adds a line noting that growth scans are usually offered every two weeks from 16 weeks, with your care team confirming the exact cadence. Chorionicity also travels into the doctor-visit summary and its PDF, so the fact you entered once turns up in the document you hand over. The question is asked for twins only, none of the cards carry risk percentages, and every one routes back to your care team.

## FAQ

**What does chorionicity mean?**

Chorionicity is the number of placentas your twins have; amnionicity is the number of amniotic sacs. Together they give you the three labels on your scan report: di/di (DCDA), mo/di (MCDA) and mo/mo (MCMA).

**What is the difference between di/di and mo/di twins?**

Di/di twins have one placenta each and one sac each. Mo/di twins share a single placenta but have their own sacs. Sharing a placenta means sharing blood vessels, which is why mo/di pregnancies are scanned more often and delivered earlier.

**How does the 11-14 week scan tell di/di and mo/di twins apart?**

The sonographer looks at where the dividing membrane meets the placenta. A thick wedge of tissue there is the lambda or twin peak sign and means di/di; a thin membrane meeting the placenta at a right angle is the T sign and means mo/di. ISUOG advises determining chorionicity before 13+6 weeks using as many features as possible, including membrane thickness and the number of placental masses.

**Does chorionicity tell me whether my twins are identical?**

Only in one direction. The NHS puts it plainly: babies who share a placenta are identical, while babies with two separate placentas can be identical or not. So mo/di and mo/mo answer the question, and di/di does not.

**What happens if the scan cannot tell which type we have?**

Guidance is deliberately cautious. ISUOG advises seeking a second opinion from a tertiary referral centre, and says that if the centre is uncertain it is safest to classify the pregnancy as monochorionic — meaning you get the closer monitoring rather than the lighter schedule.

**How often will my twins be scanned?**

For uncomplicated pregnancies, ISUOG describes a scan every two weeks from 16 weeks onwards for monochorionic twins; dichorionic twins get the first-trimester scan, the anomaly scan at around 20 weeks, and one every four weeks after that. All monochorionic monoamniotic pregnancies are referred to a tertiary centre. Your own unit sets the exact cadence.

**When are di/di, mo/di and mo/mo twins usually born?**

Guidance differs by country, so ask your own team. UK NHS unit guidance aligned to NICE NG137 aims for 37+0 to 37+6 weeks for di/di, 36+0 to 36+6 for mo/di and 32+0 to 33+6 for mo/mo. US-side StatPearls describes 38+0 to 38+6, 34+0 to 37+6 and 32+0 to 34+0 respectively.

## Sources

- ISUOG Practice Guidelines (updated): role of ultrasound in twin pregnancy — Khalil A, Sotiriadis A, Baschat A, et al. Ultrasound Obstet Gynecol 2025;65(2):253-276 — chorionicity before 13+6 weeks, lambda vs T sign, scans every 2 weeks from 16 weeks for monochorionic twins and every 4 weeks after the ~20-week anomaly scan for dichorionic, delivery 37+0-37+6 uncomplicated dichorionic and 36+0-36+6 uncomplicated monochorionic, all MCMA referred to a tertiary centre: https://pmc.ncbi.nlm.nih.gov/articles/PMC11788470/
- NHS — Pregnant with twins — A scan before 14 weeks should accurately show what kind of placenta your twins have; sharing a placenta means identical, two placentas means they may or may not be: https://www.nhs.uk/pregnancy/finding-out/pregnant-with-twins/
- StatPearls — Multiple Birth Delivery (NCBI Bookshelf NBK599524) — US-side planned delivery windows (di-di 38+0-38+6, mo-di 34+0-37+6, mo-mo 32+0-34+0); mo-di is 1 in 5 twin pregnancies; mo-mo 8 per 100,000; serial growth scans from 16 weeks: https://www.ncbi.nlm.nih.gov/books/NBK599524/
- NHS Greater Glasgow & Clyde — Twin Pregnancy Ultrasound Guideline (330) — UK unit guideline aligned to NICE NG137 — chorionicity from placental masses, membrane thickness and the lambda or T sign; manage as monochorionic if uncertain; birth 37+0-37+6 DCDA, 36+0-36+6 MCDA, 32+0-33+6 MCMA: https://www.rightdecisions.scot.nhs.uk/ggc-clinical-guidelines/maternity/ultrasound/twin-pregnancy-ultrasound-guideline-330/
- Chorionicity-associated variation in metabolic phenotype of cord blood in twins — Nutr Metab (Lond) 2023;20:31 — Chorionicity assessed from membrane thickness and the T-sign or lambda sign between 11 and 14 weeks; monochorionic twins delivered at a median of 36 weeks (IQR 35-37) vs 37 weeks (IQR 37-37) for dichorionic, p=0.02: https://pmc.ncbi.nlm.nih.gov/articles/PMC10339575/

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