If your pregnancy symptoms are earlier and more pronounced than you’d normally expect, you may be carrying a twin pregnancy. However, only a health professional can tell for sure.

1. You just feel that you are 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 have an elevated hCG level (hCG is a pregnancy related hormone). This hormone is the one associated with morning sickness.

3. Other typical pregnancy symptoms may be exaggerated. Many women — but not all — who are pregnant with twins have more intense pregnancy symptoms, probably related to the extra hormones circulating through their system. You may find that your breasts are extremely tender, you have to urinate frequently, you are hungry all the time and you are always very tired.

4. A higher than average weight gain in the first 16 weeks 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.

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. If your last menstrual period indicates an eight-week gestation, your uterus may feel more like 10 to 12 weeks to your obstetrician. This may prompt a request for an ultrasound examination.

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 spina bifida abnormalities. AFP levels are higher in twin pregnancies.

7. Your health care provider hears two fetal heartbeats. Two separate heartbeats can be distinguishable with a Doppler in your care provider’s office from 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.

8. You have a positive ultrasound. If you believe you are 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.

9. In later pregnancy, 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.

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

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:

1. Can be of different sexes and may have different blood types.

2. They may look very different from one another, with different-coloured hair and eyes. They may also look alike, as siblings often do.

3. Tend to run in families.

4. 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

1. Are always the same sex and blood type.

2. They do not necessarily look exactly alike. One may be right-handed while the other is left-handed.

3. They develop at random. They are not related to maternal age, race, or family history of twins.

4. Multiples that are conceived with infertility treatment are usually fraternal, rather than identical.

What are the causes of multiple pregnancy?

1. With increasing maternal age the chance of twins increases. To put it in perspective, your chance of giving birth to twins if you are less than 25 years of age is less than half of what it would be after the age of 35.

2. You are more likely to conceive twins in the first few menstrual cycles after stopping birth control pills.

3. 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, therefore, more likely to have a multiple pregnancy if you use fertility drugs or assisted reproductive technology (ART - In Vitro Fertilization IVF).

4. ART is the transfer of embryos directly into the uterus. 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.

What is the prevalence of multiple pregnancy?

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 and a tendency for women to deliver later in their reproductive years. 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.