Twins

Twins are two babies developing in the uterus during one pregnancy. Having twins is not a disease. It is a type of multiple pregnancy.

Twins may be dizygotic, often called non-identical or fraternal, or monozygotic, often called identical. Dizygotic twins come from two separate eggs fertilized by two separate sperm. Monozygotic twins begin with one fertilized egg that divides into two embryos.

Another important distinction is whether the babies share a placenta or amniotic sac. The placenta supplies oxygen and nutrients during pregnancy. An amniotic sac is the fluid-filled membrane around a baby.

Clinicians describe this arrangement as chorionicity and amnionicity. These details affect which complications are possible and how often ultrasound monitoring may be needed. They are not the same as whether twins are identical.

Dizygotic twins develop when two eggs are released and fertilized during the same menstrual cycle. Each baby has its own genetic combination, as other siblings do. The twins may be the same or different sexes and may look quite different.

Monozygotic twins develop when one early embryo separates. They usually share most of their DNA, but they are not necessarily identical in every physical or medical detail. Changes during development and differences in the pregnancy environment can affect each baby differently.

All dizygotic twins have separate chorions and amniotic sacs. Their placentas may sit close together and appear joined. Monozygotic twins may have separate or shared placentas and sacs, depending on when the embryo divided.

Common arrangements include:

• Dichorionic diamniotic: each baby has a separate placenta and sac
• Monochorionic diamniotic: the babies share a placenta but have separate sacs
• Monochorionic monoamniotic: the babies share both a placenta and a sac

An ultrasound can confirm that there are two babies and usually identify the placenta and sac arrangement. This is often easiest earlier in pregnancy. Later scans can assess growth, fluid around each baby, and blood flow when needed.

Ultrasound usually cannot prove whether same-sex twins with separate placentas are identical. A genetic zygosity test after birth can often answer that question if the family wants to know.

A twin pregnancy usually needs more monitoring than a pregnancy with one baby. The exact schedule is based largely on whether the babies share a placenta or sac, as well as their growth and the pregnant person's health.

Twins are more likely to be born early. Earlier birth can mean that one or both babies need extra support with breathing, feeding, temperature control, or other care after birth. This does not mean every twin pregnancy will have these outcomes.

Growth can also differ between twins. Ultrasound measurements help clinicians compare each baby's growth over time rather than relying on a single measurement.

Some pregnancy conditions, including pre-eclampsia, gestational diabetes, and iron-deficiency anaemia, are more common in multiple pregnancies. Pre-eclampsia is a pregnancy-related condition involving high blood pressure and signs that organs such as the kidneys or liver may be affected.

Sharing a placenta creates additional considerations because blood vessels from the two babies can be connected. This is why knowing the chorionicity is medically important.

Twin pregnancy can also affect decisions about where and how birth takes place. Planning considers the babies' positions, gestational age, growth, placenta arrangement, previous births, and any complications. Having twins does not automatically mean that a caesarean birth is required.

The following are conditions or situations linked with twin pregnancy. They do not occur in every case.

• Preterm birth: birth before 37 completed weeks of pregnancy
• Fetal growth restriction: one or both babies grow more slowly than expected
• Growth discordance: a notable difference between the babies' estimated sizes
• Pre-eclampsia: pregnancy-related high blood pressure with other health effects
• Gestational diabetes: high blood sugar first found during pregnancy
• Miscarriage or fetal loss: loss of one or both babies during pregnancy
• Postpartum haemorrhage: heavier-than-expected bleeding after birth

In twin-to-twin transfusion syndrome, or TTTS, blood flows unevenly between twins sharing a placenta. One baby receives less blood while the other receives more. Twin anaemia-polycythaemia sequence, or TAPS, is another shared-placenta condition that causes a difference in the babies' red blood cell levels. These conditions require specialist assessment.

Twins sharing the same amniotic sac can have problems involving their umbilical cords, including cord entanglement. This uncommon arrangement usually leads to close specialist monitoring.

Sometimes an early scan shows two embryos, but a later scan shows that only one continues to develop. This is often called a vanishing twin. What it means for the pregnancy depends partly on when it happens and whether a placenta is shared. It can also be an emotionally significant loss.

Ask what the ultrasound report says about chorionicity and amnionicity. Useful terms to record include dichorionic, monochorionic, diamniotic, and monoamniotic. If the report is unclear, ask whether another scan or specialist review is planned.

Keep a record of:

• Your estimated due date
• Whether the babies share a placenta or sac
• Each baby's label on scans, such as Twin A and Twin B
• Planned scan and antenatal appointments
• Contact details for your maternity assessment service

Attend the monitoring offered to you. It may include ultrasound scans, blood pressure checks, urine tests, blood tests, and discussions about birth. Ask what each test is checking and when you should expect the result.

There is nothing you can do to change whether twins are identical or whether they share a placenta. Do not start medicines or supplements specifically because you are carrying twins unless your maternity team recommends them for you.

Ask your maternity service when and how to monitor fetal movement. Contact them promptly if either baby's usual movement pattern seems reduced or changed. Do not wait for the next routine appointment.

Seek urgent maternity assessment for vaginal bleeding, fluid leaking from the vagina, regular painful contractions before 37 weeks, reduced fetal movement, or a severe headache with vision changes. These symptoms need prompt assessment but do not always mean that a serious problem is present.

Call emergency services for heavy bleeding, severe breathing difficulty, chest pain, a seizure, loss of consciousness, or severe and persistent abdominal pain. Routine questions about scan terms, appointment timing, nutrition, or birth preferences can wait for your next appointment unless your care team advises otherwise.

Do twins run in families?

A family tendency can increase the chance of dizygotic twins because releasing more than one egg may be inherited. Maternal age and fertility treatment can also affect the chance. Monozygotic twinning is usually not explained by family history.

Are identical twins always the same sex?

Monozygotic twins are usually recorded as the same sex at birth because they began from one fertilized egg. Rare genetic or developmental differences can occur.

Does one placenta always mean identical twins?

Twins who truly share one placenta are usually monozygotic. However, two separate placentas can lie together and look like one. Ultrasound features, rather than appearance alone, are used to assess the arrangement.

Can twins be born vaginally?

Some twins can be born vaginally, while others are safer to deliver by caesarean birth. The plan is based on the babies' positions and health, the placenta arrangement, the stage of pregnancy, and the pregnant person's circumstances.

Can both babies be breastfed?

Many families breastfeed or express milk for twins, sometimes alongside formula. Feeding support can help with positioning, milk expression, and a plan that fits the health of both babies and the parent.

Will twins have the same health needs after birth?

Not necessarily. Even twins with very similar DNA may differ in size, feeding, development, or medical needs. Each baby should receive individual newborn checks and ongoing care.