Twins can be the same (identical-monozygotic) or different (fraternal-dizygotic). Fraternal twins, triplets, or more develop when two or more eggs are fertilized by separate sperm. Fraternal fetuses have separate placentas and amniotic sacs (the bag containing the fluid around a baby).

Identical twins come from a single egg that has been fertilized by one sperm. For unknown reasons, the fertilized egg splits into two or more embryos during the first stage of development. Some identical twins share the same placenta. However, they usually grow within separate amniotic sacs in the uterus. In rare cases, identical twins share one amniotic sac (monoamniotic).

Fraternal multiples:

  • Can be of different sexes and may have different blood types.
  • May look very different from one another, with different-coloured hair and eyes. They may also look alike, as siblings often do.
  • Tend to run in families.
  • They are more common in older mothers, African races and when there is a family history of twins. They are least common in Asian families. If you have already carried fraternal twins, you have double the usual risk of having twins. A history on your partner's side of the family does not increase your risk of multiple pregnancy.

Identical twins

  • Are always the same sex and blood type.
  • They do not necessarily look exactly alike. One may be right-handed while the other is left-handed.
  • They develop at random. They are not related to maternal age, race, or family history of twins.
  • Multiples that are conceived with infertility treatment are usually fraternal, rather than identical.


  • With increasing maternal age the chance of twins increases. To put it in perspective, your chance of giving birth to twins if you are under 25 years of age is less than half of what it would be after the age of 35.
  • You are more likely to conceive twins in the first few menstrual cycles after stopping birth control pills.8901 9301
  • Infertility treatment. Fertility drugs stimulate your ovaries to release multiple eggs, which can be fertilized at once. This may result in a multiple pregnancy. You are more likely to have a multiple pregnancy if you use fertility drugs or assisted reproductive technology (ART - In Vitro Fertilization  IVF).
  • ART involves the transfer of embryos directly into the uterus - embryo transfer. In the early days, when success rates were extremely low, several embryos were transferred in the hope of achieving a pregnancy. Nowadays, 25% to 30% of pregnancies from ART are twins; 5% are triplets and less than 1% are quadruplets or more. As techniques have improved, success rates from IVF/embryo transfer have increased and reasonable success rates are being achieved with single embryo transfer and this reduces the incidence of multiple pregnancies.


The natural incidence of twins is 1 in 80 pregnancies and for triplets 1 in 8000 pregnancies. The incidence is rising as a result of IVF,9401 0301 and a tendency for women to deliver later in their reproductive years.0301 Naturally occurring quadruplet births occur in 1 per 600,000 births. The UK would, therefore, expect one set of naturally occurring quadruplets each year. There are estimated to be approximately 125 million human twins and triplets in the world, and just 10 million identical twins.

One in two pregnancies with same sex babies are monozygous (identical twins). The rate of identical twins remains at about 1 in 333 pregnancies across the globe.



  • Pregnancy symptoms, such as nausea, vomiting, heartburn, insomnia and fatigue, abdominal pain, shortness of breath and pressure on the pubic bone are common  during pregnancy and are increased when there is more than one baby.
  • Each additional fetus increases your risk of miscarriage, premature birth,0201 gestational diabetes, pre-eclampsia, placenta abruption, placenta praevia, urinary tract infections, anaemia, caesarean delivery, pulmonary embolism, and heavy blood loss after delivery (postpartum haemorrhage).0401 
  • A many as 1 in 8 pregnancies start out as multiples, but only a single fetus is brought to full term, because the other has died very early in the pregnancy and not been detected or recorded. Early obstetric ultrasonography exams sometimes reveal an "extra" fetus, which fails to develop and instead disintegrates and vanishes. This is known as vanishing twin syndrome. Identical twins may share the afterbirth. As a result one twin may have an increased blood supply and the circulation to the other is reduced. This is a twin-to-twin transfusion. This may require planned early delivery. The perinatal mortality is increased in monozygous twins.08-01
  • There is an increased perinatal mortality (risk of a baby dying around the time of childbirth) for twins three to ten-fold 8201, 8701, 8801, 9101, 0102 and twelve-fold for triplets0102, 0302

Because they are frequently born prematurely, multiples also have a higher-than-usual rate of problems after birth. A typical, single pregnancy lasts about 40 weeks, but a twin pregnancy often lasts between 35 to 37 weeks. Nearly half of all twins are born prematurely (before 37 weeks), and the risk of having a premature delivery increases with higher-order multiples. Many will require only routine newborn care, while those with significant prematurity or other complications may require high-level intensive care in specialized centres.

Intrauterine growth retardation: birth weights of twins, and higher order multiple pregnancies, are smaller than the weights of corresponding singletons. However, when combined, birth weights of twins are greater than weights of corresponding singletons. Most of the deficit of birth weight occurs in the final 8-11 weeks of pregnancy. Average birth weights are similar between twins and singletons until 32 weeks of gestation. Average birth weights are similar between triplets and singletons until 29 weeks of gestation. Birth weight discrepancies of more than 20-25% are considered discordant. Discordant birth weights occur in 10% of twins.

The prevalence of congenital abnormality is doubled for twin babies. 8601, 8901, 9901 Amniocentesis and chorionic villus sampling (CVS), which are invasive tests to check for defects, are associated with a five-fold risk of miscarriage in twin pregnancy compared to singletons 9301, 0101

There is no evidence that prophylactic beta-mimetics reduce the incidence of premature labour for twins.0802


How is a multiple pregnancy managed?

How can I tell that I have a multiple pregnancy?

If your  pregnancy  If your prenancy symptoms  are earlier and more severe than you'd normally expect, you may be carrying a multiple pregnancy. However, only a health professional can tell for sure.

1 You just feel you're carrying more than one baby. Some mothers of twins (or higher order multiples) say that they knew right from the start that they were carrying more than one baby.

2. You experience more nausea and/or morning sickness. If you are having more than one baby, you may also have an elevated hCG level, a pregnancy related hormone. Higher levels of hCG also make it more likely that you will have bouts of morning (or all-day) sickness.

3. Other normal pregnancy symptoms may be exaggerated. Many women -- but not all -- who are pregnant with twins have more intense pregnancy symptoms, likely due to the extra hormones circulating through their system. You may find that your breasts are very tender, you have to urinate frequently, you are hungry all the time and you are very tired. In the second trimester, you may experience difficulty catching your breath, swelling (oedema) of the hands and legs, an unusual rate of weight gain and abdominal enlargement and excessive fetal movement. Anaemia or low iron (decreased haemoglobin) is also common with twin and multiple pregnancies.

4. You gain weight rapidly in your first trimester. A higher than average weight gain in the first trimester may be your first clue that you're carrying more than one baby. Good weight gain in early twin pregnancy is associated with a favourable outcome. 9801

With each additional fetus a woman carries, the range of weight gain will increase. For example, a woman who starts a pregnancy in a healthy weight range might expect her weight gain to be as follows:

One fetus, 11 kg (24 lb) to 16 kg (35 lb)

Twins, 16 kg (35 lb) to 20 kg (44 lb)

Triplets, 20 kg (44 lb) to 23 kg (51 lb)

Quads, 23 kg (51 lb) to 25 kg (55 lb)

5. Your uterus seems large for gestational age. At your first exam you may be told that your uterus is "large for dates." If your last menstrual period indicates an eight-week gestation, your uterus may feel more like 10 to 12 weeks. This may prompt your care provider to request an ultrasound.

6. You are told that you have elevated levels of AFP. AFP is one of the chemicals tested to screen for Down?s syndrome and and spina bifida abnormalities. Levels of alpha fetoprotein (AFP), a protein released by the baby as it grows and found in the mother's blood, can be elevated when there is more than one baby.

7. You are told you have rapidly rising hCG levels. Human Chorionic Gonadotrophin (hCG) is a hormone produced by the fertilized egg and by the chorionic villi (early placenta). It is needed to maintain the pregnancy until the placenta fully develops. It can be detected in your blood or urine even before you miss a period. Normally in a singleton pregnancy, blood (serum) concentrations of hCG rise rapidly during the first weeks, doubling every two to three days. Levels of hCG can be even higher with twin or multiple pregnancies.

8. Your health care provider hears two fetal heartbeats. Two separate heartbeats can be distinguishable with a Doppler in your care provider's office by around 18 weeks. At around 28 weeks, it may be possible to differentiate two fetal heads and multiple small parts when doing an abdominal exam.

9. You have a positive ultrasound. If you believe you are indeed pregnant with twins, an ultrasound can be performed quite early in pregnancy. With a skilled ultrasonographer, two gestational sacs, two embryos and two distinct fetal heartbeats can be seen six weeks after the first day of the last menstrual period. Many twins have been diagnosed as early as five weeks -- when you're just one week late for your menstrual period. 

A fetal ultrasound can show a picture of the inside of the uterus on a computer screen. This is the most accurate way to learn whether you have more than one fetus.

There is considerable disagreement among medical professionals as to whether a woman carrying twins is more likely to feel movement earlier. But many moms do claim that those early flutters and flips were their first clue that they might be having more than one.


All multiple pregnancies are considered high-risk. If you are carrying more than one fetus, you will have more frequent medical checks than you would with only one fetus. For example, you might be seen every two weeks from 26 to 34 weeks and then weekly. There is an increased chance that you will deliver your babies early, so make sure you and your health professional review the signs of labour that begins early (preterm labour) and what you should do when you go into labour.

It is always important to take extra good care of yourself when pregnant. When carrying twins or more (multiple pregnancy), be sure to eat a balanced and nutritious diet of quality calories, and make sure that you get enough calcium, iron, and folic acid. Your doctor is likely to prescribe additional folic acid.

Tests used to monitor the fetuses and mother during a multiple pregnancy

Blood pressure checks at every prenatal appointment are used to monitor you for high blood pressure or pre-eclampsia.

Blood testing is used to check you for low iron (anaemia). Anaemia is a common problem for women with multiple pregnancy because the fetuses use a great amount of the mother's iron stores.

A urine test and urine culture can be used to screen for a urinary tract infection (UTI).

Transvaginal ultrasound can be used to check the length of your cervix. A short cervix is a sign of an increased risk of preterm labour.

Ultrasound examination is usually performed at four week intervals to check that your babies are growing adequately

Electronic fetal heart monitoring may be used before or during delivery to make sure the fetuses are doing well.

About 50 percent of women carrying twins deliver their babies by caesarean section ? especially if the babies aren't in a head-down position. Caesarean sections are considered safer for triplets and higher order multiples.

The babies may be affected because of premature birth, with problems for the lungs, brain, heart, and eyes. There may be long-term disability for the babies including mental retardation, cerebral palsy, learning disabilities, blindness, or deafness. These are most likely among babies born before 28 weeks. There is no evidence that bed rest and home labour monitoring can prevent premature labour.

Breastfeeding multiples is certainly possible and many mothers of twins and even triplets are successful in breastfeeding all of their babies.

Multifetal Pregnancy Reduction


When there are three or more fetuses in the uterus, their risks of disability or death are higher with each additional fetus. If you are carrying triplets or more your doctor may offer the option of multifetal pregnancy reduction (MFPR) near the end of your first trimester. A successful MFPR increases the chances of healthy survival for the remaining fetuses and reduces risks to you. However, MFPR sometimes leads to miscarriage.

The decision to have a multifetal pregnancy reduction is difficult and traumatic. If you are faced with this decision, talk to your doctor about your personal risks of trying to carry multiple fetuses to term versus choosing MFPR. Also consider discussing your decision with a counsellor or spiritual advisor.

Women's Health


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