OpenMFM · MFM Patient Education

Your Twin Pregnancy with
One Shared Placenta

Understanding monochorionic–diamniotic (MCDA) twins — why you are monitored closely, what the extra ultrasounds look for, and when delivery is planned

Identical Twins Ultrasound Every 2 Weeks Delivery 36–37 Weeks
Atlanta Perinatal Associates Maternal-Fetal Medicine
Understanding Your Twins

Your Babies Share One Placenta
but Have Two Separate Sacs

Monochorionic (mono = one, chorion = placenta) — your twins share a single placenta.  |  Diamniotic (di = two, amnion = sac) — each baby has its own protective fluid sac, separated by a thin dividing membrane.

One Shared Placenta

One placenta nourishes both babies. Tiny blood vessels connect them through it — which is why your care team watches these twins more closely than twins who have separate placentas.

Two Separate Sacs

Each baby has their own amniotic sac and their own personal space. We always check the fluid on both sides of the dividing membrane.

🔍 Your team determines the chorionicity (how the placentas and membranes are arranged) and amnionicity — ideally in the first trimester. This early answer tells us exactly how closely to monitor your pregnancy. (ACOG)
Source: ACOG · SMFM · ISUOG
Atlanta Perinatal Associates Slide 2 of 17
Why We Watch More Closely

Why MCDA Twins Need Extra Monitoring

The American College of Obstetricians and Gynecologists (ACOG) recommends that monochorionic–diamniotic twins be followed more closely than dichorionic twins (twins with separate placentas), because the shared placenta carries higher risks of:

Twin-to-Twin Transfusion Syndrome (TTTS)

An imbalance of blood flow between the twins through shared placental vessels.

Twin Anemia-Polycythemia Sequence (TAPS)

A subtler blood imbalance — one twin anemic, the other with too many red blood cells.

Fetal Anomalies & Heart Differences

Congenital anomalies — including congenital heart disease — are more common in monochorionic twins.

Stillbirth in Late Pregnancy

The risk is higher than for twins with separate placentas, especially in the final weeks.

✅ Here is the reassuring part: all of these conditions can be found on ultrasound — often before symptoms ever appear. Regular, expert surveillance is exactly what lets your team act early and protect both babies.
Atlanta Perinatal Associates Slide 3 of 17
Early Pregnancy Plan

First Steps: Confirm the Type of Twins
and Check the Heart

First Trimester

Confirm the due date and determine chorionicity and amnionicity — ideally at the first-trimester scan. This sets the intensity of your entire monitoring plan.

Detailed Anatomy Survey

ISUOG and the updated SMFM checklist both call for a detailed anatomy survey — including first-trimester anatomy — looking carefully at each baby’s structures.

Fetal Echocardiogram — 18–22 Weeks

Because congenital heart disease is more common in monochorionic twins, ACOG recommends a detailed fetal echocardiogram (specialized heart ultrasound) at 18–22 weeks.

💡 A fetal echo is recommended because MCDA twins have a somewhat higher chance of heart differences — not because a problem is expected. For most families, the scan is completely normal.
Source: ACOG · SMFM (updated MC/DA checklist) · ISUOG
Atlanta Perinatal Associates Slide 4 of 17
Your Monitoring Schedule

Ultrasound Every Two Weeks,
Starting Around 16 Weeks

Serial ultrasound begins about every 2 weeks starting around 16 weeks and continues until delivery — the schedule ACOG, SMFM, and ISUOG recommend for MCDA twins.

📋 Growth is assessed about every 4 weeks in uncomplicated pregnancies (per the updated SMFM checklist), while the 2-week visits continue for fluid, bladder, and Doppler checks. Every other week is the sweet spot — long enough for changes to appear, short enough to catch them early.
Diagram: original summary of the ISUOG surveillance pathway · Source: ACOG, SMFM, ISUOG
Atlanta Perinatal Associates Slide 5 of 17
Inside Each Visit

What We Check at Every Ultrasound —
and How Doppler Is Used

  • Amniotic fluid on both sides of the dividing membrane (the deepest pocket)
  • Each baby’s bladder — a sign the kidneys are working
  • Size measurements (biometry) of both babies
  • Growth plot — formal growth assessment every 4 weeks
  • Doppler blood flow — added when it can guide care
  • MCA-PSV screening for TAPS — considered from 16 weeks (SMFM)

Umbilical Artery Doppler (the cord)

Checks how well blood flows from the placenta to each baby. Routine cord Doppler is not beneficial when growth and fluid are normal — it is used when the babies differ in size or fluid. (ACOG)

MCA-PSV (a vessel in the brain)

Measures blood-flow speed in each baby’s brain. Faster flow can signal anemia — this is how we screen for TAPS, considered from 16 weeks on. (SMFM)

💡 You may not see Doppler at every visit — and that is expected. The team adds Doppler when it can actually answer a question about your babies’ health.
Source: ACOG · SMFM Consult Series #72
Atlanta Perinatal Associates Slide 6 of 17
The Condition We Watch For Most

Twin-to-Twin Transfusion Syndrome (TTTS)

In TTTS, blood moves unevenly through the shared placental vessels — from one twin (the “donor”) into the other (the “recipient”). It affects about 10–15% of MCDA pregnancies.

The Donor Twin — Too Little Fluid

Gives away blood, so urine and amniotic fluid drop. The diagnosis is made when the deepest fluid pocket is less than 2 cm (oligohydramnios).

The Recipient Twin — Too Much Fluid

Receives extra blood, so urine and fluid rise. The diagnosis requires the other sac’s deepest pocket to be more than 8 cm (polyhydramnios).

⚠️ Before making the diagnosis, your team rules out other causes — selective fetal growth restriction and structural, genetic, or infectious problems — so the right care plan is chosen.
✅ TTTS is found on ultrasound — usually before you feel any difference. Fluid pockets and bladders are checked at every visit, which is exactly why the two-week schedule works.
Source: ACOG · SMFM Consult Series #72
Atlanta Perinatal Associates Slide 7 of 17
If TTTS Develops

TTTS Is Grouped Into Stages —
and Laser Treatment Is the Standard

Once TTTS is diagnosed, ultrasound findings such as bladder filling and fluid extremes group it into stages (I–IV). The stage guides your care team’s recommendations:

Stage I — the mildest stage

Usually managed expectantly — with surveillance at least weekly. Laser surgery may be considered if you develop symptoms from too much amniotic fluid (polyhydramnios).

Stages II–IV

Fetoscopic laser surgery is the standard treatment when TTTS presents between 16 and 26 weeks. The laser seals the connecting vessels on the placenta — a recommendation carrying the strongest evidence grade (1A).

🏥 If you are eligible for laser treatment, your team will refer you to a fetal intervention center — a specialized unit experienced in this procedure. You will be guided through the entire process, step by step.
👶 Caught early and treated at a fetal center, TTTS has far better outcomes than untreated disease. Most families who need laser therapy go on to welcome healthy babies.
Source: SMFM Consult Series #72
Atlanta Perinatal Associates Slide 8 of 17
Another Blood Imbalance

Twin Anemia-Polycythemia Sequence (TAPS)

TAPS is a slower, quieter imbalance than TTTS: one twin becomes anemic (too few red blood cells) while the other develops polycythemia (too many red blood cells — thicker blood). Because it develops gradually, TAPS often shows no fluid swings — so it is found by Doppler, not by symptoms.
Twin Blood State MCA-PSV Finding (Doppler)
Donor twin Anemia — too few red blood cells Brain blood flow faster than >1.5 MoM
Recipient twin Polycythemia — too many red blood cells Brain blood flow slower than <1.0 MoM
Between twins — Difference between the twins >0.5 MoM

“MoM” (multiples of the median) simply compares each measurement with the normal value for that week of pregnancy — a way of saying “higher or lower than expected.”

💡 Because MCA-PSV screening for TAPS may be part of your scans from 16 weeks, this condition is usually caught early — when your team has the most options to respond. (SMFM)
Source: SMFM Consult Series #72 · TAPS Delphi consensus criteria
Atlanta Perinatal Associates Slide 9 of 17
Growth Differences

When One Twin Grows More Slowly (sFGR)

Because the twins share one placenta, one baby can receive a smaller share of blood and grow more slowly — called selective fetal growth restriction (sFGR). Management is individualized using the Gratacós classification, based on the smaller twin’s umbilical artery Doppler pattern:

Type I

Cord blood flow stays steady and forward. Generally the most stable pattern — expectant management with close ultrasound follow-up is appropriate.

Type II

Blood flow is persistently absent or reversed at rest. Carries higher morbidity — may warrant fetal therapy and very close surveillance.

Type III

Blood flow alternates unpredictably between normal and absent/reversed. Also higher risk — the plan is individualized and may include fetal therapy.

💡 Size alone does not decide the plan — the Doppler pattern does. Your MFM team uses the type to decide how closely to watch and whether a fetal treatment center should be involved. (Gratacós classification)
Source: Gratacós et al. classification · ACOG · ISUOG
Atlanta Perinatal Associates Slide 10 of 17
Rarer Situations

Other Conditions That Are Unique
to a Shared Placenta

TRAP Sequence

Twin reversed arterial perfusion is an uncommon complication in which one twin’s heart does not form properly, and blood flows in a reverse direction through the shared circulation. Evaluation and management are individualized, usually with a fetal center.

Loss of One Twin

If one baby passes away during the pregnancy, the shared blood vessels can affect the surviving co-twin — with a risk of death or neurologic injury. Rapid expert evaluation and careful surveillance are essential in this situation.

🤝 These situations are uncommon, and they are included so you understand why specialist teams respond quickly and carefully if anything ever changes. You will never face these decisions alone — and no amount of worry can prevent them, but monitoring is what lets us respond best.
Source: ACOG · SMFM · ISUOG
Atlanta Perinatal Associates Slide 11 of 17
Late-Pregnancy Checks

Adding Weekly Checks at 32 Weeks

For an otherwise uncomplicated MCDA pregnancy, weekly antenatal fetal surveillance may be considered beginning at 32 0/7 weeks — because the risk of stillbirth is higher in the final weeks of these pregnancies.

32 0/7
weeks — weekly fetal checks may begin
↓
stillbirth risk in monitored MCDA twins
≈
risk approaches that of singletons at the same gestational age
✅ These weekly visits are part of why late-pregnancy outcomes for closely monitored MCDA twins are so encouraging — the monitoring lowers stillbirth rates toward the level of singletons at similar gestational ages.
Source: ACOG · SMFM · ISUOG
Atlanta Perinatal Associates Slide 12 of 17
Delivery Planning

When Will Your Babies Be Born?

Uncomplicated MCDA twins are delivered before the due date, because the shared placenta carries a small but real risk in the final weeks. Experts agree on the window — your team picks the timing within it:

SMFM 2026 Checklist
34 0/7 – 37 6/7
weeks — the delivery window SMFM recommends for uncomplicated MCDA twins
ISUOG Optimal Window
36 0/7 – 36 6/7
weeks — going beyond this may increase the risk of perinatal death
💡 In practice, an uncomplicated MCDA pregnancy is usually delivered in the 36–37 week range — inside the SMFM window and at ISUOG’s optimal timing. If a complication develops, your MFM team may adjust the plan earlier and will always explain why.
Source: SMFM (updated MC/DA checklist) · ISUOG Practice Guidelines
Atlanta Perinatal Associates Slide 13 of 17
How Your Babies Will Be Born

Mode of Delivery & Medications
That Support Your Pregnancy

Vaginal Birth May Be an Option

In uncomplicated MCDA twins with a vertex (head-down) presenting twin, vaginal birth may be considered at 32 weeks or later — regardless of the second twin’s presentation — when an experienced obstetrician is available and decisions are made together with you. (SMFM)

Cesarean When Medically Indicated

A cesarean delivery is used when it is the safest choice — for obstetric reasons or your babies’ needs. Your team will discuss the plan and the reasons with you as delivery approaches.

Low-Dose Aspirin

Routine low-dose aspirin to prevent preeclampsia is part of standard care for many monochorionic twin pregnancies.

Antenatal Corticosteroids

Given in anticipation of planned early delivery, these shots speed up your babies’ lung maturity before birth.

Magnesium Sulfate

For neuroprotection — protecting your babies’ developing brains — when delivery is anticipated before 32 weeks.

✅ You do not need to memorize any of this — your team will explain each medication before it is given and why it is recommended at that moment.
Source: SMFM (updated MC/DA checklist) · ACOG · ISUOG
Atlanta Perinatal Associates Slide 14 of 17
You Are Part of the Team

Your Role — and Questions to Ask Your MFM Specialist

Your Part of the Plan

  • Attend every ultrasound visit — even when you feel completely fine
  • Keep taking your prenatal vitamins and eating well
  • Rest when your body asks — twin pregnancy is hard work
  • Call early with concerns — don’t wait to wonder
  • Write down questions between visits

Questions Worth Asking

  • What type of twins do I have, and when was that confirmed?
  • Which checks are part of my every-2-week ultrasound?
  • What would a change in fluid or Doppler mean for my babies?
  • If TTTS, TAPS, or a growth difference is found — what are our options?
  • When is delivery planned — and what could move it earlier?
  • Is a vaginal birth an option for me, and who would be present?
📱 Call your care team right away if you notice vaginal bleeding, leaking fluid, regular cramping or contractions, a sudden change in your belly, shortness of breath, or decreased movement from either baby. These symptoms don’t always mean a problem — but they deserve a prompt check.
Atlanta Perinatal Associates Slide 15 of 17
Summary

The Bottom Line

1
shared placenta — the reason for closer monitoring
2 wks
ultrasound visits from ~16 weeks until delivery
36–37
weeks — planned delivery window when uncomplicated
2
healthy babies — the goal of every visit
  • MCDA twins share one placenta — so ACOG recommends they are watched more closely than other twins
  • Ultrasounds every 2 weeks from ~16 weeks screen for TTTS, TAPS, and growth differences
  • TTTS caught early can be treated — often with laser surgery at a fetal intervention center
  • Weekly fetal checks may be added from 32 weeks, lowering late-pregnancy risk toward singleton levels
  • Uncomplicated MCDA twins are delivered between 34 0/7 and 37 6/7 weeks — optimally at 36–37 weeks
  • Most closely monitored MCDA pregnancies end with two healthy babies
🤝 You and your babies are in expert hands. Attend every visit, ask every question, and trust the schedule — it exists for one reason: to bring both of your babies home safely.
Information consistent with ACOG, SMFM, ISUOG, and current evidence-based guidelines.
This material supports — and does not replace — a conversation with your physician.
Atlanta Perinatal Associates Maternal-Fetal Medicine
Where This Information Comes From

Sources & Guidelines

  • 1 American College of Obstetricians and Gynecologists (ACOG). Multifetal Gestations: Twin, Triplet, and Higher-Order Multifetal Pregnancy. Practice Bulletin No. 231 (2021) — basis for closer MCDA monitoring, fetal echocardiography at 18–22 weeks, serial ultrasound from ~16 weeks, and Doppler guidance. View source
  • 2 Society for Maternal-Fetal Medicine (SMFM). Consult Series #72: Twin-Twin Transfusion Syndrome and Twin Anemia-Polycythemia Sequence (2024) — surveillance elements, TTTS staging and laser therapy (evidence grade 1A), and MCA-PSV screening for TAPS from 16 weeks. View source
  • 3 Society for Maternal-Fetal Medicine (SMFM). Updated MC/DA twin management checklist (2026) — delivery window 34 0/7–37 6/7 weeks, vaginal birth considerations at ≥32 weeks with a vertex presenting twin, first-trimester anatomy, and 4-weekly growth assessment in uncomplicated pregnancies.
  • 4 ISUOG Practice Guidelines (updated): Role of ultrasound in twin pregnancy. Ultrasound in Obstetrics & Gynecology, 1 February 2025 — intensified surveillance pathway and the optimal delivery window of 36 0/7–36 6/7 weeks for uncomplicated monochorionic twins. View source Figure 3 of the ISUOG guideline is summarized as an original diagram on Slide 5. ISUOG content used under license from Wiley.
  • 5 Gratacós E, et al. A classification system for selective intrauterine growth restriction in monochorionic pregnancies according to umbilical artery Doppler flow in the smaller twin. Ultrasound in Obstetrics & Gynecology 2007;30:28–34. View source
  • 6 TAPS consensus criteria. Consensus diagnostic criteria and monitoring of twin anemia-polycythemia sequence: Delphi procedure (2020).
Guideline recommendations change over time — your care team’s plan may differ slightly from this summary, and that is appropriate for your individual pregnancy.

Privacy: OpenMFM educational presentations do not request, collect, or store any personal health information.
Prepared by Dr. Chukwuma Onyeije, MD · Atlanta Perinatal Associates · OpenMFM.org
Atlanta Perinatal Associates Maternal-Fetal Medicine
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