TAPS, or Twin Anemia Polycythemia Sequence, is a rare complication that only affects pregnancies with identical twins who share a single placenta. In TAPS, blood slowly transfers from one twin to the other through tiny blood vessel connections in the placenta, leaving one twin anemic (too few red blood cells) and the other with too many red blood cells. It occurs in up to 5% of these pregnancies spontaneously, and can also develop after laser surgery for a related condition called twin-to-twin transfusion syndrome (TTTS).
How TAPS Develops
Identical twins who share a placenta (called monochorionic twins) often have blood vessels that connect their two circulatory systems within the placenta. In most cases, blood flows back and forth relatively evenly. In TAPS, however, there are only a few of these connections, and they’re extremely small, less than 1 millimeter in diameter. Because the connections are so tiny, the blood transfer happens very slowly, over weeks or even months rather than days.
Over time, this slow, one-directional trickle moves red blood cells from the “donor” twin to the “recipient” twin. The donor gradually becomes anemic, meaning their blood can’t carry oxygen as efficiently. The recipient accumulates excess red blood cells, making their blood thicker than normal. This thickened blood is harder for the heart to pump and can cause its own set of problems. Both twins can develop cardiovascular strain if the imbalance becomes severe enough.
How TAPS Differs From TTTS
TAPS and twin-to-twin transfusion syndrome are sometimes confused because both involve unequal blood sharing between twins. The key difference is what gets transferred and how it shows up on ultrasound. In TTTS, the imbalance is primarily about fluid volume. One twin produces too much amniotic fluid (polyhydramnios) while the other has dangerously little (oligohydramnios). This fluid discrepancy is the hallmark of TTTS and is usually obvious on a standard ultrasound.
In TAPS, both twins typically have normal amniotic fluid levels. The problem is specifically a large gap in red blood cell counts between the two babies. This makes TAPS harder to detect with routine ultrasound alone, since the most visible sign of trouble in twin pregnancies, unequal fluid, is often absent. Specialized blood flow measurements are needed to catch it.
How TAPS Is Detected
Because amniotic fluid levels look normal in TAPS, doctors rely on Doppler ultrasound to screen for it. This type of ultrasound measures how fast blood flows through a specific artery in each baby’s brain, called the middle cerebral artery. The speed of blood flow in this artery reflects how many red blood cells a baby has. When a baby is anemic, blood flows faster because thinner blood moves more quickly. When a baby has too many red blood cells, blood flows more slowly because it’s thicker.
Doctors look for a specific pattern: the donor twin’s blood flow speed measures above 1.5 times the expected value for their gestational age, while the recipient’s measures below 1.0 times the expected value. This combination has very high specificity for TAPS, meaning when both thresholds are met, the diagnosis is essentially certain. However, the sensitivity is lower (around 46%), which means some milder cases can be missed by these strict cutoffs. Newer approaches using the difference in blood flow speed between the two twins may improve early detection.
Current guidelines recommend that all monochorionic twin pregnancies undergo ultrasound monitoring every two weeks starting at 16 weeks of gestation. This regular schedule gives doctors the best chance of catching TAPS and other complications before they become severe.
Spontaneous vs. Post-Laser TAPS
TAPS arises in two different scenarios. Spontaneous TAPS develops on its own in about 5% of monochorionic twin pregnancies. It tends to appear later in pregnancy and progress gradually because the tiny vessel connections that cause it allow only a slow trickle of blood transfer.
Post-laser TAPS occurs in 2% to 16% of cases after fetoscopic laser surgery for TTTS. During laser treatment for TTTS, a surgeon seals off the abnormal blood vessel connections between the twins. If a few very small connections are missed during the procedure, those remaining vessels can become the pathway for TAPS to develop afterward. This is why twins who’ve had laser surgery for TTTS are monitored closely in the weeks that follow.
Staging and Severity
Once diagnosed, TAPS is classified using the Leiden Staging System, which ranges from stage 1 (mildest) through stage 5 (most severe). The staging evaluates blood flow patterns and cardiovascular symptoms in both twins, helping doctors gauge how urgently treatment is needed. Early-stage TAPS may be managed with close monitoring alone, while higher stages indicate that the blood imbalance is putting one or both babies at significant risk and intervention should be considered.
Treatment Options
There is no single standard treatment for TAPS, and the best approach depends on the stage, the gestational age, and how quickly the condition is progressing. The main options include:
- Close monitoring: For early-stage TAPS detected well before the babies are mature enough for delivery, frequent Doppler ultrasounds (often weekly or more) track whether the blood imbalance is stable or worsening. Some cases of stage 1 TAPS remain stable for weeks without intervention.
- Intrauterine blood transfusion: If the donor twin becomes significantly anemic, a blood transfusion can be delivered directly to the baby through the umbilical cord while still in the womb. This corrects the anemia temporarily but doesn’t address the underlying cause, so repeat transfusions may be needed.
- Fetoscopic laser surgery: A thin camera is inserted into the uterus to identify and seal off the tiny blood vessel connections causing the imbalance. This is the only treatment that targets the root cause of TAPS. It is technically challenging because the vessels involved are so small, but it can stop the transfusion permanently when successful.
- Early delivery: When TAPS develops late in pregnancy or progresses rapidly, delivering the babies early may be the safest option. The timing balances the risks of prematurity against the risks of worsening blood imbalance.
What TAPS Means for the Babies
The outlook for babies affected by TAPS depends heavily on when it’s caught and how severe it becomes. Mild cases detected early and managed with regular monitoring often result in healthy outcomes for both twins. More advanced stages carry greater risks, particularly for the anemic donor twin, who may develop heart strain from trying to compensate for low oxygen-carrying capacity.
After birth, the donor twin may need blood transfusions to correct persistent anemia, while the recipient may need treatment to thin their overly concentrated blood. Both babies are typically monitored in a neonatal unit after delivery. Long-term neurodevelopmental outcomes are an area of ongoing attention for all monochorionic twin complications, though severe outcomes like hearing loss or vision problems appear to be uncommon. In one large follow-up study of monochorionic twins treated with laser surgery, bilateral deafness occurred in only 0.4% of cases, and blindness was not observed.
The most important factor in outcomes is early detection through consistent screening. If you’re carrying monochorionic twins, the biweekly ultrasound schedule starting at 16 weeks is specifically designed to catch conditions like TAPS before they reach advanced stages.