Your growth scan report says one baby is smaller than the other. Maybe the word on the page is “discordance.” Maybe your doctor said, “Let’s keep an eye on it.” Either way, you are probably searching for one answer: is this serious?

unequal growth in twin pregnancy is common, and many twins have small differences that never cause a problem. But a larger gap, or a gap that keeps growing, needs a proper explanation. The explanation depends mostly on one thing: whether your twins share a placenta.

First, What Does “Unequal Growth” Actually Mean?

Doctors estimate each baby’s weight from measurements taken during the scan: head size, abdomen size and thigh bone length. They then compare the two babies.

The difference between the two weights, shown as a percentage of the larger baby’s weight, is called twin growth discordance.

As a rough guide:

• Under 15 to 20% is usually considered a normal variation between two individuals
• 20% or more deserves closer attention
• 25% or more, especially along with other findings, is a stronger warning sign

Two things matter as much as the number:

1. Is the smaller baby small for its age? A baby who is smaller than the other but still growing within the normal range is very different from a baby whose weight falls below the 10th centile (smaller than 9 out of 10 babies of the same age).
2. Is the gap stable or widening? A difference that stays the same across scans is far less worrying than one that grows.

A scan weight is also an estimate, not an exact figure. It can be off by a noticeable margin, which is why one reading is never enough. The trend across serial scans tells the real story.

Why Does One Twin Grow Slower?

There is no single cause. The reasons fall into a few groups.

1. Unequal sharing of the placenta

This is the most common reason, and it is especially important in twins who share one placenta (monochorionic twins). Think of the placenta as a plot of land with a fixed boundary. If one baby’s cord is attached to a larger share of it, that baby gets more nourishment. The other baby gets less, and grows more slowly.

The same thing can happen in twins with two separate placentas if one placenta is smaller or does not work as well as the other.

2. Position of the cord

The cord does not always attach in the centre of the placenta. If one baby’s cord is attached at the edge, or to the membranes rather than the placenta itself, blood flow to that baby can be less efficient.

3. Blood vessel connections in a shared placenta

When twins share a placenta, tiny blood vessels connect their circulations. These are normal, but the balance between them differs from pregnancy to pregnancy. Sometimes the balance tips and affects one baby more. This can lead to a related condition called twin-to-twin transfusion syndrome (TTTS), covered in our guide on twin pregnancy stage by stage.

4. A difference in the babies themselves

Occasionally, one baby is smaller because of a chromosomal or genetic condition, a structural problem, or an infection that affected only that baby. This is less common than placental causes, but it is why the smaller baby always gets a detailed anatomy check.

5. Maternal factors

Conditions like high blood pressure, diabetes, anaemia or kidney problems can affect growth in both babies, and sometimes affect one more than the other.

One thing worth saying clearly: unequal growth is not caused by something you ate or did not eat, by resting too little, or by stress. The cause is almost always in how the placenta is shared, not in anything a mother did. If you have been blaming yourself, please stop.

Why Placenta Sharing Changes Everything

Here is the point that decides your whole plan.

Twins with separate placentas (dichorionic). If one twin is smaller, the cause is usually that baby’s own placenta. The babies are not connected, so the smaller baby’s problem does not directly affect the bigger one. Monitoring is usually growth scans every few weeks, with extra checks if the gap widens.

Twins who share a placenta (monochorionic). The babies are connected through shared blood vessels. So a problem with one baby’s share of the placenta can affect the other baby too. This is why shared-placenta twins are scanned every two weeks from about 16 weeks, and why unequal growth in this group is taken more seriously.

This is also why confirming the placenta type early in pregnancy, ideally before 14 weeks, matters so much. If you are not sure what type of twins you are carrying, a detailed assessment through advanced fetal ultrasound services can help, and a second opinion fetal ultrasound is a sensible step if your report is unclear or the findings have been explained poorly.

Selective Fetal Growth Restriction (sFGR): The Condition to Know About

When one twin is significantly small, doctors may call it selective fetal growth restriction, or sFGR. In general terms, this means one baby is clearly growth restricted while the other is growing normally.

Doctors usually suspect it when:

• One baby’s estimated weight is below the 3rd centile, or
• Two or more of the following are present: the smaller baby’s weight is below the 10th centile, its abdominal size is below the 10th centile, the weight difference between the twins is 25% or more, or the blood flow in its umbilical cord is abnormal

Why blood flow (Doppler) matters

A routine scan shows size. A Doppler scan shows how well blood is flowing through the umbilical cord to each baby. In shared-placenta twins with sFGR, this is the most important test, because the pattern of flow decides how closely the pregnancy needs watching.

In very simple terms, doctors group shared-placenta sFGR into three patterns:

• Type I: Blood flow in the smaller baby’s cord is still normal in the resting phase of the heartbeat. This is the mildest pattern, and these pregnancies often continue well for a long time.
• Type II: Blood flow is persistently absent or reversed. This needs the closest monitoring, because the smaller baby is more likely to worsen.
• Type III: Blood flow switches back and forth between normal and absent or reversed. It can look stable and then change quickly, so it needs careful and frequent checks.

This is why a simple “one baby is smaller” report is never the full picture. The Doppler pattern is what tells your doctor how often you need scans and when delivery should be planned.

How Is It Monitored?

Monitoring depends on the placenta type and the Doppler findings. In general:

• Growth scans are repeated at regular intervals to track both babies and the gap between them
• Doppler studies of the umbilical cord, and sometimes other vessels, are used to assess the smaller baby
• Fluid levels around each baby are checked, because a big difference between the two sacs can point to a different condition
• Cervical length may be measured, since twins are at higher risk of early labour
• Your own blood pressure and urine are checked more often

In shared-placenta twins with sFGR, scans may become weekly or even more frequent if the Doppler pattern is concerning.

Complex cases often need more than one specialist working from one plan. That is the idea behind multidisciplinary fetal care, where fetal medicine, neonatology and other specialists share the decisions instead of giving you separate opinions.

Can Anything Be Done Before Delivery?

For many pregnancies, careful monitoring and well-timed delivery is the plan. But in some shared-placenta pregnancies, treatment during pregnancy is possible.

Depending on the findings, options discussed by fetal medicine specialists can include:

• Close surveillance with a pre-planned delivery date (the most common approach)
• Laser treatment of the shared placental vessels, in selected situations where there is also transfusion between the babies
• Selective procedures in rare, carefully chosen cases where one baby’s condition threatens the survival of the other

Which, if any, of these applies is a specialist decision, based on the exact findings. A consultation for fetal anomaly and therapy is where these options are explained with real numbers, including what each option can and cannot achieve. Nobody should be pushed toward a procedure, and nobody should be told “nothing can be done” without a proper specialist assessment.

If the smaller baby’s growth problem raises a concern about a genetic cause, prenatal genetics and genetic counselling can explain what testing makes sense, and when invasive prenatal diagnostic testing is worth considering. Families who first want to understand what earlier results meant can read through the available pregnancy screening tests.

When Is Delivery Planned?

There is no single date. It depends on:

• Whether the twins share a placenta
• The Doppler pattern in the smaller baby
• How both babies are growing, and whether the gap is widening
• Your own health

As a general idea, mild patterns in shared-placenta twins are often followed until the mid-to-late 30s of weeks, while more worrying Doppler patterns can lead to earlier delivery, sometimes in the early 30s. For twins with separate placentas and a stable difference, delivery is often closer to the usual twin timing.

These are planning ranges, not rules. The point of close monitoring is to choose a delivery time that balances two risks: leaving the smaller baby too long in a placenta that is struggling, and delivering both babies too early.

Because early delivery is more likely in these pregnancies, it helps to know in advance which hospital will manage the birth, and that a neonatal unit is available on site. Planning this properly is part of multiple pregnancy care.

What You Can Do

You cannot fix a placental cause by changing your diet. But you can make sure the pregnancy is managed well.

1. Know your placenta type. Ask whether your twins are monochorionic or dichorionic, and get it in writing.
2. Ask for the numbers. What is each baby’s estimated weight, what is the percentage difference, and what did the Doppler show?
3. Ask for the follow-up plan. How often are scans needed, and what findings would change the plan?
4. Attend every scan. In shared-placenta twins especially, the timing of scans is part of the treatment.
5. Eat well and take your iron, calcium and prescribed supplements. This will not close the gap, but it supports both babies and protects your own health.
6. Do not delay if something feels different. Reduced movements of either baby, sudden swelling, severe headache or any bleeding or fluid leakage should be checked the same day.

When to See a Fetal Medicine Specialist

A specialist opinion is worth taking if:

• The weight difference is 20% or more, or is increasing between scans
• One baby is below the 10th centile
• The twins share a placenta
• The fluid levels differ between the two sacs
• The Doppler report shows any abnormal flow
• You have had a previous growth-restricted baby, preterm birth or pregnancy loss
• You have high blood pressure, diabetes or another medical condition

These situations are the focus of high risk pregnancy services, and the specialty behind them is maternal-fetal medicine.

Frequently Asked Questions

Is it normal for one twin to be smaller than the other?

A small difference in size is common and often normal. Two babies are two individuals, and they rarely weigh exactly the same. A difference of 20% or more, or a smaller baby falling below the 10th centile, needs a closer look.

What is a normal weight difference between twins?

There is no single normal figure, but differences under about 15 to 20% are usually considered within normal variation. Doctors also look at whether the gap is stable or increasing, and at the Doppler findings.

Does the bigger baby take food from the smaller one?

In twins with separate placentas, no. Each baby has its own supply. In twins who share a placenta, the share of the placenta each baby has access to can differ, and that can affect growth, but this is about placenta anatomy, not one baby “stealing” from the other.

Can I do anything to make the smaller baby grow?

Eating well and taking prescribed supplements supports your health and both babies, but it cannot change how the placenta is divided. Bed rest has not been shown to correct the problem. The most useful step is proper monitoring and a specialist plan.

Is the smaller twin going to be born with problems?

Many smaller twins do well, especially when the Doppler flow is normal and delivery is timed carefully. Risks are higher when blood flow is abnormal or delivery has to be early, which is why the monitoring plan matters.

How often will I need scans?

It depends on the placenta type and the Doppler pattern. Shared-placenta twins are scanned every two weeks from about 16 weeks, and more often if there is selective growth restriction. Separate-placenta twins with a growth difference are usually scanned every two to four weeks.