We continue with our research on how and why twins are conceived. Days ago we described the types of twins and now we will focus on the chances of having twins.
Before starting to expose point by point the different circumstances that can influence when designing twins, let's clarify that we talk about probabilities. If you are within any of these assumptions you are more likely to conceive twins, but you also may not have them even if all conditions exist. The same as a woman with zero probabilities can have multiple ovulation and conceive twins.
Now, we will talk especially about the chances of having bicigotic or bivitine twins, which if you have read the previous post are two babies conceived at the same time independently, that is, two ovules fertilized by two sperm cells, what we know as twins. We will refer to the probabilities related to double ovulation (or triple, or quadruple, etc.).
Probabilities of having bicigotic or biviteline twins (twins)
Mother's weight
There are investigations (American College of Obstetrics and Gynecology) that have related a Body Mass Index (BMI) of 30 or higher it would be a factor with a tendency to poliovulation (ovulate more than one ovule per cycle) and therefore, with a higher probability of having twins.
A higher probability of conceiving twins has also been found in tall women, but this theory has less strength than that related to the mother's weight.
Mother's age
The women over 35 they have greater possibilities of a multiple conception. Due to hormonal causes there is an increase in ovarian follicular stimulation causing the follicles to produce multiple ovules. This means that women who are elderly mothers are more likely to have twins than young mothers. It is paradoxical, because although at these ages there is less chance of pregnancy, hormonal imbalances that occur towards the end of the fertile age make double ovulation more feasible in the same menstrual cycle. It is estimated that 70% of women over 45 have twins.
Family heritage
Having a history of multiple pregnancies in the family increases the chances of having them. 17 percent of twins have twins. This inheritance is transmitted both maternally and paternally, but it is expressed in women, of course, men do not ovulate. That is, women who have twins in their family (by ascending inheritance) will be more likely to have them than women who have no history of twins in the family.
In the case that the father has a family history of twins, that greater probability will be transferred to his daughter (if he has one). The myth of skipping a generation is not true, but in this case it could happen.
Having had twins before
The propensity linked to the number of births. Having had twins in a previous birth double the chances of conceiving them again. It would be based on a mother's tendency to multiple ovulation. As with other issues that are repeated in different pregnancies of the same mother, having had a twin pregnancy means that she can have it again.
Mother's race
The evidence shows that there is a highest percentage of twins among the black race, followed by Caucasian, Hispanic and Asian.
Assisted and in vitro reproduction techniques
Nowadays more and more children are born through assisted reproduction techniques due, among other factors, to the delay of maternity.
These techniques usually resort to ovulation stimulation to increase the chances of conception, therefore also increases the chances of having twins.
Also, in vitro fertilization, two to three embryos are implanted in the mother to ensure the chances of at least one thriving and a baby being born. But in the event that the two or three embryos thrive, it will result in a multiple pregnancy.
If 2 embryos are transferred, the rate of twin gestations is approximately 30 percent, while if 3 embryos are transferred, the chances increase to 50 percent.
Contraceptives
The possibilities are minimal, but the anovulatory contraceptive methods (birth control pill) can lead to a twin pregnancy. There is a 1% chance that the method will fail and ovulation of one or more ovules occurs. Likewise, being an ovulation inhibitor, if the treatment is not used as it should be or conceived soon after leaving it, there could be a rebound effect stimulating ovulation and produce a multiple pregnancy.
Probabilities of having monozygotic or univithelial twins (identical twins)
In the event that the twins were the result of an egg fertilized by a sperm that, when divided, resulted in two fetuses, what are known as identical twins, the factors that influence this to occur are not as identified as in the case of the factors that influence poliovulation in the twins.
The cause that causes an embryo to duplicate its genetic material to become two identical parts in its initial phase is not known for sure.
When looking for information about the factors that influence to conceive identical twins I have found two versions. The one that indicates that univithelial twins are independent of genetic inheritance, which are the product of genetic chance, and another that suggests a slight probability of genetic inheritance, that is to say that in families that have born identical twins there is a greater probability that this repeat
In addition, it has been found a greater probability to have identical twins related to the in vitro fertilization technique. It is not known why, but in addition to promoting the birth of twins, in vitro treatment increases the possibility that the zygote divides after implantation in the uterus and leads to a monozygotic twin pregnancy.
There are couples with chances of having twins who did not want to have them and others who wanted to increase their chances of having them. In any case, it is something that couples, at the moment, cannot decide because the limits between chance and science are unknown.
Photos | Flickr cybaea, TheNickster
More information | uam
In Babies and more | Types of twins
