What are the chances?
Since the 1980's the incidence of twins and multiples has increased dramatically with the most recent figures suggesting than approximately 150 out of every 10,000 births are multiples with a third of these being identical and two thirds being fraternal.
What's the difference between identical and fraternal twins?
Genetically, fraternal (or dizygotic) twins are only as similar as siblings. This is because they come from two separate eggs that have been fertilised by two separate sperm and simply developed in the same womb at the same time. On the other hand, identical (or monozygotic) twins share the same genetic makeup as they come from a single egg that split during the first few days of the pregnancies development.
What affects the chances of having twins and multiples?
- The increased advancement and availability of fertility treatments have played an active role in making twin and multiple births more common. This is because many of these treatments either stimulate the ovaries to produce eggs (making it more likely that more than one egg is matured and released during each menstrual cycle) or, as with IVF, two or three eggs are implanted into the womb, again allowing the potential for more babies to develop at the same time.
- Age also influences the likelihood of multiple births with 'older Mums' being significantly more likely to conceive twins than those in their twenties. This is thought to be because women's bodies produce increasing amounts of the hormones that stimulate ovulation as they grow older, making it more likely that more than one egg will be released per cycle.
- Fraternal twins are said to run in families, being passed down genetically on the mother's side. This could potentially be down to a gene for hyper-ovulation that's passed through the generations making it more likely that a woman will release more than one egg during ovulation. Identical twins do not seem to run in families.
- Research has suggested that maternal height and weight plays a role in the likelihood of multiple births with taller women and those with a BMI of 30 or more being more likely to have more than one baby.
How will I know if i'm having twins?
As nurturing a multiple pregnancy places more strain on your body, many women report experiencing stronger pregnancy symptoms earlier on, with many having more extreme fatigue and morning sickness. However, others sail through the early weeks without any trouble at all so this is not a definite indicator by any means.
Greater weight gain during the first trimester can also be a sign but again this varies from woman to woman and may simply be due to a miscalculation of due dates rather than a multiple pregnancy.
The only sure way to find out if you're expecting twins or more is by having an ultrasound, so if you're not there yet you could get a big surprise when you have your first scan!
Identical or fraternal, when do you find out?
If you're expecting more than one baby it's natural to want to find out whether they'll be identical or not. However, it's likely that you will have to wait a little while to find out.
If your first scan clearly shows that you are expecting two or more babies that are being nourished by a single placenta it's safe to say that you're carrying identical twins. However, if two placentas can be seen then it's not possible to tell whether your twins will be identical or not.
Later scans may give you more of a clue as identical twins will be the same sex. So, if you are clearly having one boy and one girl then your babies will be just as similar as brother and sister genetically.
Once your babies arrive their DNA can be tested to look for genetic symmetry, but while this is a definite way to find out whether your twins are identical or fraternal it does usually come at a cost. However, as your babies grow and their physical characteristics develop you'll soon be able to figure it out on your own.
Whether you are expecting, twins, multiples or just the one, why not visit the AskBaby forums and share your questions about pregnancy, birth and beyond with other members.