DCDA (dichorionic diamniotic) twins each have their own placenta and amniotic sac. MCDA (monochorionic diamniotic) twins share one placenta but have separate sacs. The shared placenta in MCDA pregnancies creates specific risks, particularly Twin-to-Twin Transfusion Syndrome (TTTS), and requires fortnightly ultrasound monitoring from 16 weeks. DCDA pregnancies are monitored every four weeks. Chorionicity is best determined by ultrasound before 14 weeks.
When you find out you are expecting twins, one of the first things your care team will want to establish is your chorionicity: how many placentas your babies share. This single piece of information shapes your entire pregnancy care plan, your scan schedule, and the specific risks your care team will be watching for.
Most parents who get the twin diagnosis hear 'DCDA' or 'MCDA' within minutes of the scan. No one fully explains what it means until you go looking. DCDA. MCDA. MCMA. Monochorionic. Dichorionic. This article explains what each classification means, how it is determined, and what it means in practical terms for your pregnancy.
What chorionicity means and why it matters more than anything else
Chorionicity refers to the number of chorionic sacs, which is effectively the number of placentas. A dichorionic pregnancy has two chorionic sacs: each baby has their own placenta. A monochorionic pregnancy has one chorionic sac: both babies share a single placenta.
Amnionicity refers to the number of amniotic sacs: the fluid-filled sacs directly surrounding each baby. Most twin pregnancies, whether monochorionic or dichorionic, have two amniotic sacs (diamniotic). The rarer monoamniotic pregnancies have only one, shared sac.
Whether your babies share a placenta determines the complications they can and cannot develop, and how often your care team needs to check on them.
DCDA twins: two placentas, two sacs
DCDA (dichorionic diamniotic) is the most common twin type, accounting for approximately 70% of all twin pregnancies. Each baby has their own placenta and their own amniotic sac. The two placentas may fuse together and appear as one on ultrasound, particularly later in pregnancy, but they are functionally separate.
DCDA twins can be fraternal (dizygotic), arising from two separately fertilised eggs, which is the most common cause. They can also be identical (monozygotic) if a single fertilised egg splits very early, in the first three days after fertilisation, before the chorion has differentiated. This is less common.
Because each baby has their own blood supply through their own placenta, DCDA twins cannot develop complications that arise from a shared placental circulation. TTTS is not possible in a DCDA pregnancy.
According to NICE guidelines (NG137, 2019), uncomplicated DCDA pregnancies are monitored with growth scans approximately every four weeks from the dating scan. Planned delivery for uncomplicated DCDA pregnancies is typically offered at 37 to 38 weeks.
MCDA twins: one placenta, two sacs
MCDA (monochorionic diamniotic) pregnancies account for approximately 27–28% of twin pregnancies. Both babies share one placenta but each has their own amniotic sac, separated by a thin membrane. MCDA twins are always identical (monozygotic): the single fertilised egg split between days 4 and 8 of development, after the chorion had already begun to form as one unit.
The shared placenta in MCDA pregnancies means the two babies' circulations are connected through blood vessels on the placental surface. In most cases this arrangement is balanced. When it is not, two complications specific to monochorionic pregnancies can develop:
- Twin-to-Twin Transfusion Syndrome (TTTS): unequal blood transfer between the twins through shared placental vessels. Affects approximately 10–15% of MCDA pregnancies (ISUOG guidelines). Detectable by ultrasound through fluid discordance between the two sacs.
- Selective intrauterine growth restriction (sIUGR): one twin receives a disproportionately small share of the placenta and grows significantly more slowly than the other. Requires close monitoring of Doppler blood flow measurements.
Because these complications can develop and progress rapidly, NICE guidelines (NG137, 2019) require fortnightly ultrasound monitoring for all MCDA pregnancies from 16 weeks of gestation. Planned delivery for uncomplicated MCDA pregnancies is typically offered at 36 to 37 weeks.
Know your twin type. Track your monitoring schedule.
Twin Hearts adapts everything to your chorionicity. MCDA or DCDA, the week-by-week content, AI guidance, and scan reminders are tailored to your specific pregnancy.
MCMA twins: one placenta, one sac
MCMA (monochorionic monoamniotic) pregnancies are rare, accounting for approximately 1% of twin pregnancies. Both babies share a single placenta and a single amniotic sac, with no membrane separating them.
The absence of a separating membrane means the umbilical cords can become entangled, which is the primary risk in these pregnancies. MCMA pregnancies require close specialist monitoring, typically in a tertiary fetal medicine centre with experience managing this rare twin type.
Planned delivery for MCMA twins is usually offered at around 32 to 34 weeks by elective caesarean section, to reduce the risk of cord-related complications in late pregnancy.
How chorionicity is determined: what to look for on your scan
Chorionicity is best determined by ultrasound in the first trimester, ideally before 14 weeks. This matters because the ultrasound signs that distinguish dichorionic from monochorionic pregnancies are clear and reliable before 14 weeks but become progressively harder to identify after that point.
The two signs your sonographer is looking for where the membrane between the twins meets the placenta:
- The lambda sign (twin peak sign): a thick, triangular wedge of placental tissue projecting into the base of the inter-twin membrane. This indicates a dichorionic pregnancy. The "peak" of placental tissue grows into the gap between the chorionic layers as the placenta expands.
- The T-sign: a thin, T-shaped junction where the membrane meets the edge of the placenta, with no placental tissue projecting between the layers. This indicates a monochorionic pregnancy.
ISUOG practice guidelines for multiple pregnancy and NICE guidelines (NG137, 2019) both emphasise that chorionicity should be determined and documented as early as possible in pregnancy. If you have your first scan after 14 weeks, or if the sign was not clearly documented at your dating scan, ask your sonographer or midwife whether chorionicity has been formally recorded in your notes.
What to ask your care team
Before your next appointment, confirm two things: whether chorionicity has been formally documented in your notes, and what your scan schedule looks like from here. If you are carrying MCDA twins, ask when fortnightly monitoring begins and at which unit. If DCDA, ask about your growth scan schedule and planned delivery timing. If a fetal medicine referral has not been mentioned, it is reasonable to ask whether one is planned.
For MCDA pregnancies in particular, your care may be shared between your local maternity unit and a fetal medicine specialist. Knowing where each element of your care happens, and who to contact if you are concerned between appointments, is worth establishing early.
Frequently asked questions about DCDA and MCDA twins
DCDA and MCDA FAQ
What is the difference between DCDA and MCDA twins?
DCDA (dichorionic diamniotic) twins each have their own placenta and their own amniotic sac. MCDA (monochorionic diamniotic) twins share one placenta but have separate amniotic sacs. The shared placenta in MCDA pregnancies creates additional risks, including Twin-to-Twin Transfusion Syndrome (TTTS), and requires fortnightly ultrasound monitoring from 16 weeks.
How is chorionicity determined?
Chorionicity is most reliably determined by ultrasound before 14 weeks. A dichorionic pregnancy shows the lambda (twin peak) sign: a thick triangular wedge of placental tissue at the base of the inter-twin membrane. A monochorionic pregnancy shows the T-sign: a thin T-shaped junction with no placental tissue projecting between the layers. These signs become harder to identify after 14 weeks, which is why early scanning matters.
How often are scans for DCDA twins vs MCDA twins?
According to NICE guidelines (NG137, 2019), DCDA twin pregnancies are monitored with growth scans approximately every four weeks from the dating scan. MCDA pregnancies require ultrasound monitoring every two weeks from 16 weeks of gestation, due to the risk of TTTS and other complications associated with a shared placenta.
Are MCDA twins always identical?
Yes. Monochorionic twins are always genetically identical. A shared placenta only occurs when a single fertilised egg splits after the chorion has already begun to form as a single unit, meaning both babies develop within the same outer sac. DCDA twins can be either fraternal or identical, depending on when the split occurred.
What complications are MCDA twins at risk of that DCDA twins are not?
The most significant complication exclusive to MCDA pregnancies is Twin-to-Twin Transfusion Syndrome (TTTS), which occurs due to unequal blood flow through shared vessels on the placental surface. MCDA pregnancies are also at higher risk of selective intrauterine growth restriction (sIUGR). Neither condition can affect DCDA twins, who have separate placentas with no shared circulation.
What is MCMA and how is it different from MCDA?
MCMA (monochorionic monoamniotic) twins share both a single placenta and a single amniotic sac, with no membrane separating them. This occurs in approximately 1% of twin pregnancies. The absence of a separating membrane creates a risk of cord entanglement, which is the primary concern. MCMA twins are typically delivered earlier, at around 32 to 34 weeks, usually by planned caesarean section.
Twin Hearts knows your twin type
Whether you are carrying DCDA or MCDA twins, Twin Hearts tailors your week-by-week content, AI midwife guidance, and scan reminders to your specific chorionicity. Both parents have full access, in real time.