Ultrasound (8-14 weeks)

More often than not, the ultrasound is the first indication that there are two babies in the womb. An ultrasound image will show if there is more than one foetus. The first scan (usually around eight to 14 weeks) will find a twin pregnancy in nearly all cases. Very occasionally a twin can hide but you'd be incredibly unlucky if the second scan in the second or third trimester didn't pick it up.

An early ultrasound that indicates twins can lead to disappointment. You'll have no sooner got over the news and adapted to the idea of two babies when the subsequent scan reveals only one baby. This is known as "Vanishing Twin Syndrome" and occurs in the first crucial weeks of pregnancy. The second sac simply vanishes, is reabsorbed or is miscarried. Usually by around 12 wks it is possible to tell whether or not both sacs have signs of life, and it is around then when you will find out that you really are having twins or not. It is likely that many single pregnancies started out as twin pregnancies with the second sac having vanished before the first ultrasound, leaving mum none the wiser.

Doppler heartbeat count (late first trimester)

During your antenatal checks, the doctor or midwife routinely listens to baby's heartbeat using a Doppler system. If there is more than one baby you will be able to hear more than one heartbeat, indicating a multiple pregnancy.

Levels of HGC hormone (first trimester)

About 10 days post-conception, levels of the hormone HCG rise in the body. Over-the-counter pregnancy tests measure the levels of HCG in the urine to indicate whether a woman is indeed pregnant or not.

The level increases at a rapid rate, peaking at about 10 weeks into the pregnancy. Twins may produce an elevated level of HCG. An elevated result in a pregnancy test carried out by the doctor (where the level is quantified rather than the simple yes / no result of an over-the-counter test) may indicate twins but is often put down to confusion over dates. An ultrasound will help confirm both.

Abnormal AFP test results (15 - 17 weeks)

At around the 15th - 17th week of gestation you will be offered the alphafetoprotein (AFP) test. This is a screening test where blood is taken and the levels of AFP are measured to help identify babies who suffer from neural tube defects (NTD) or Down Syndrome.

A twin pregnancy can produce an unusually high or "positive" result which can cause a great deal of anxiety in a mother who is at that stage still unaware she is carrying twins. Generally, an ultrasound will be done for further assessment.

Fundus height and weight (second / third trimester)

Whilst most mums expecting twins do put on a bit more weight than those with only one baby, excessive weight gain is generally the result of overeating.

At antenatal classes the midwife measures the height of the uterine fundus (from the top of the pubic bone to the top of the uterus). This shows that the baby is growing normally and can be measured against the gestational age. A twin or multiple pregnancy may causes the mother's uterus to expand beyond the range of a single pregnancy. However, other factors may also increase the measurements. In the past, before the sophistication of ultrasound and screening blood tests, this was often the best indication that a mum was expecting more than one baby.

Early or frequent foetal movement (16 - 22 weeks)

Feeling those first little flutters of the baby (or babies) moving inside the womb is one of the most thrilling aspects of pregnancy. Although some mums with multiple pregnancies appear to experience more frequent or earlier foetal movement, some medical professionals argue that there is no scientific evidence of this.