Twin pregnancy: symptoms, types and risks

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It occurs with a probability of one in 80 cases, certainly has a hereditary component and is associated with high rates of maternal and fetal complications. But a woman expecting twins has every chance to experience pregnancy peacefully if she is well informed and well accompanied

“You won’t believe it, they are expecting twins!” The news of the arrival of two twins has always been experienced by future parents, relatives and friends as an extraordinary event; and for good reason, since in humans the twin pregnancy spontaneously is a pretty rare event . In addition to the charm and mystery, however, this topic has always generated concern for the possible maternal and fetal problems that it can entail. Let’s try to understand more.

The “causes” of twin pregnancy

As reported by the scientific literature, it is believed that the probability of a twin pregnancy is 1/80 and that this frequency decreases exponentially with the increase in the number of embryos.
Twins can come from a single egg and a single sperm (monozygotic or “identical twins”) or from two eggs fertilized by two sperms (dizygotes or “twin brothers”).

It is believed that the splitting of the zygote in the very early stages of development is a random phenomenon, and this would explain the fact that the frequency of births of monozygotic twins (about 4 per 1,000 births) is almost constant over time and in the various populations.

Twinning certainly has a hereditary component : this would explain both the recurrence of twin births in some families, and the different frequency of twin births between different ethnic groups. We know, for example, that the twinning rate is highest among African populations and lowest among Asian ones, while Caucasian populations (to which Europeans belong) are in the middle. It has also been observed that the frequency of monozygotic births remains stable over time and in the various populations. This suggests that if there is a hereditary component, it should affect the dizygotic twins .

The results of a research that compared the DNA of mothers of dizygotic twins, mothers of monozygotic twins (all conceived without the aid of treatments) and mothers of non-twins were recently published in the American Journal of Human Genetics. . The research has identified two genetic variants that appear most often in mothers of dizygotic twins and which have to do with the production of the hormone that stimulates the maturation of follicles in the female ovaries and which can result in the release of more eggs. Then a hereditary component is confirmed that concerns only the dizygotic twins and which is linked to the mother’s genetics. It is very interesting to note that the study in question was carried out by pooling the databases of three registries of twins (the Netherlands, Australia and the United States). Regarding the “generational leap”, although there have been several studies that have sought confirmation, there is no evidence, if not anecdotal, in this sense.

Twin pregnancy and assisted fertilization

Since 1975 there has been an increase in the incidence of multiple pregnancies in various countries . This phenomenon is related to the use of couple fertility treatments , especially those that involve the transfer of two or more embryos. All this, together with the age of pregnancy which, especially in Western countries, is increasingly postponed (twin pregnancy is more frequent in women over the age of 35) and the increase in the Body Mass Index (BMI, i.e. body mass index) in the general population, outlines a challenging health picture that requires adequate tools for good management.

Contrary to what is believed by public opinion, most iatrogenic twin pregnancies (i.e. due to medical treatment) result from the simple induction of ovulation and not from the use of assisted reproductive techniques. In addition, in vitro fertilization followed by IVF(technique in which the “conceived” embryo in a test tube is transferred to the uterus to be implanted) implements a moderate control of the risk factors of multiple pregnancy, by limiting the number of embryos or oocytes that are transferred, giving rise to pregnancies low order twins (twin or triplet). On the contrary, the induction of ovulation or superovulation (the production of a greater number of oocytes than normal) not followed by oocyte pick-up (a technique that allows you to collect and therefore have a greater number of eggs available than normal natural ovulation), always present, at least theoretically, an uncontrollable risk of multiple pregnancy which is now at the center of the attention of operators for gynecological, obstetric, 

Twin pregnancy: the risks

Twin pregnancy is associated with high rates of maternal and fetal complications . The major clinical problem concerns prematurity, which as a complication is increased by about 50% in pregnancies deriving from MAP  techniques compared to spontaneous ones.

For the mother, twin pregnancy is associated with an increased risk of complications . The main ones are hypertension, Gestational diabetes, postpartum haemorrhage and the need for a caesarean section.
In the next paragraph we will analyze the risks in more detail, depending on the different types of twin pregnancy.

Types of twin pregnancy

The distinction of multiple pregnancies on the basis of the number of embryos, whether or not each one has a placenta and its own amniotic sac, is particularly important since the associated risk percentages depend on these characteristics.

In fact, we can classify multiple gestations on the basis of the number of fertilized oocytes: monozygotic , when a single oocyte is fertilized by a spermatozoon with subsequent separation of “identical twins” (1/3 of cases); dizygotic (2/3 of cases) when two oocytes are fertilized by two spermatozoa , obtaining “twin brothers”.
Next to the number of fertilized oocytes it is important to place the criteria of chorionicity (number of chorion, or rather of placentas ) and of amnioticity (number of amniotic sacs).

Monochorionic and bicorial twin pregnancy

In a monozygotic pregnancy, the number of chorion depends on when the embryo split:

  1. Biamniotic monozygotic twin pregnancy . If the cleavage occurs before the fifth day of conception, each twin will have its own placenta and amniotic sac (18-36% of cases).
  2. Twin pregnancy monozygotic monochorionic biamniotica . If the cleavage occurs approximately from the fifth to the tenth day of conception, the twins will have only one placenta and two amniotic sacs (60-70% of cases).
  3. Twin pregnancy monozygotic monochorionic monoamniotic . If the cleavage occurs after the 10th and 14th day of conception, the twins will have only one placenta and one amniotic sac (only 2% of cases).

The dizygotic twin pregnancy, on the other hand, is always biamniotic .
Compared to dizygotic pregnancies, monozygotic pregnancies present greater risks , which increase in the presence of a single placenta (monochoriality).
In light of what has been said so far, it is evident how important it is that, in the case of a twin pregnancy, the woman undergoes more intense and specialized checks than patients with single pregnancy.

Regarding the risks of prematurity and growth retardation associated with twin pregnancy, the guidelines agree on monthly ultrasound checks in case of dizygotic pregnancy and ultrasound checks every 15 days in case of monozygotic pregnancy. For this type of checks it is always good to refer to specialized centers where the staff is attentive to all aspects related to the management of the complexity and criticalities that may intervene.

Twin pregnancy: symptoms

In the case of a twin pregnancy, the classic initial symptoms of gestation may be earlier and more pronounced than in a single pregnancy. Already from the sixth week it is possible to feel the typical nausea, associated with drooling (increased salivation) which can be very annoying or disabling. In case of nausea it is always good to contact your doctor or midwife, to find strategies that can alleviate this symptom. Sometimes even just changing your diet, for example eliminating foods that contain sugar or fat, or eating little at shorter time intervals can reduce the symptom. If nausea or vomiting persists and makes it difficult to ingest food, it is important to talk to your doctor.

Other common symptoms of pregnancy, such as fatigue, sleep, change in taste and sudden mood swings, may also be more pronounced in a twin pregnancy.

But remember that the symptoms are not a sure sign of twin pregnancy ! In fact, some women with single pregnancy show symptoms of twin pregnancy, while others, expecting twins, spend the first weeks and the first trimester of pregnancy without any symptoms or only with some hint.

Twin pregnancy: when is it seen?

The only certain data for a diagnosis of twin pregnancy is the ultrasound  which, as the guidelines tell us, in the absence of complications must be performed around the eleventh-twelfth week of pregnancy. There is no reason to have an ultrasound before this period because it would not give any miscarriage prevention benefits and it would not say anything important for the management of the first few weeks of pregnancy.

For information, the presence of twins is already seen around the fifth-sixth week of pregnancy, when, as in single pregnancies, pregnancy in utero is revealed through the presence of the gestational chamber with an embryo and the yolk sac inside. If the pregnancy is twins, two gestational chambers will be seen and, in the case of a dizygotic or monochorionic biamniotic pregnancy, two embryos.
An early ultrasound is also not recommended because between the sixth and twelfth week of pregnancy about 21-30% of twin pregnancies turn into single ( vanishing twin ) .

The diagnosis of chorionicity and amnionicity of twin pregnancy, on the other hand, is recommended before the fourteenth week , and more precisely between the eleventh and thirteenth week. 

Twin pregnancy: Beta hcg dosage

The chorionic gonadotropin is a hormone , whose fractions (Beta HCG) when found in the blood or maternal urine indicates the presence of a pregnancy. This hormone is produced, in fact, by the trophoblast , that is, by the first cells that will give rise to the placenta. It is this hormone that is researched in common pregnancy tests , both those made in the laboratory and those bought at the pharmacy. 

In the case of a twin pregnancy, the Beta hcg values will be significantly higher than in a single pregnancy. The serial dosage (two samples taken 48 hours apart) of this hormone was widely used in the past to monitor the initial evolution of pregnancies, since its value doubles significantly after a few hours and each hormone value corresponds to a certain week of pregnancy, even if the reference ranges are very wide. 

The use of the Beta hcg dosage , however, has no utility demonstrated by studies and is now an antiquated practice that does not improve the outcome of those initial pregnancies, even twins, which become complicated and lead to spontaneous abortion. 

As already mentioned, in the absence of particular symptoms, it is not necessary to have an ultrasound or specific examinations before the 11th-12th week of pregnancy . This is because it is often not possible to define the evolution of pregnancy due to the early age and above all because there is no effective therapy or prevention for the threat of abortion. Therefore, the checks before this period are useless, if not to increase anxieties and worries.

Twin pregnancy and screening test

Twin pregnancy, as we have said, must be followed by a specialized and experienced gynecologist, especially for the execution of ultrasound scans. But a woman expecting twins has every chance to experience pregnancy peacefully if she is well informed and well accompanied. A mother who is expecting twins will have to face a path that is only a little more intense than the others, with some steps on which it will be necessary to pay more attention than a single pregnancy. Zero five Uppa’s manual on the health and education of children from 0 to 5 yearsby AA.VV.

An example above all is represented by the possibility of performing the screening test for Down syndrome (or combined test) which, in the case of twin pregnancy, loses some of its sensitivity, even if the screening method remains more effective. The latest studies on Non-invasive Prenatal Testing (NIPT) or fetal DNA research on maternal blood, on the other hand, are encouraging even in the case of twin pregnancy.

The chapter on prenatal diagnosis is a very delicate part in the case of twin pregnancy, because in the case of twin brothers there is the possibility of having a healthy and a sick fetus. These are extreme and very complex cases but it is necessary to be aware before embarking on the path, and to rely on specialized centers.

Some maternal pregnancy diseases, such as hypertension or gestational diabetes, are more frequent in the case of twin pregnancy but good information, good nutrition, monthly ultrasound scans every 15 days, micronutrient supplementation and rest are the basis for being able to live this period peacefully with the lowest possible risk of developing complications.

Twin pregnancy and weight gain

Growth of the belly during a twin pregnancy become more apparent from the second quarter forward. The weight gain in twin pregnancies may be greater than in single pregnancies and depends, as in the latter, on the mother’s BMI before pregnancy. 

In normal weight women with twin pregnancy, weight gain ranging from 17 to 25 kg is considered normal . A fundamental role, also in terms of improving maternal-fetal outcomes, is played by the contribution of macro and micronutrients. This is true in general and even more so in the case of twinning, so it is important that the intervention of a nutritionist is also included in the assistance program.

Twin pregnancy: childbirth

When does a twin pregnancy end? Term checks at the chosen birth point must be booked based on the progress and chorionic nature of the pregnancy and the growth of the two children.

An uncomplicated biamniotic bicorical pregnancy does not require early cardiotocographic checks while the other types of twin pregnancies must provide for early checks even of many weeks if, for example, we are talking about monochorionic monoamniotic pregnancies.

In the case of uncomplicated twin pregnancy, in order to prevent the onset of adverse events, delivery is indicated:

  • in bi-chorionic biamniotic pregnancies at 38 weeks of pregnancy ;
  • in monochorionic biamniotic pregnancies between the 36th and 37th week of pregnancy ;
  • in monochorionic monoamniotic pregnancies from 32nd to 34th week of pregnancy .

During the entire period of twin pregnancy and in view of childbirth, which can take place by caesarean section or spontaneously, as for example in the case of biamniotic bicoreal pregnancy with both cephalic fetuses, it would be good to always keep a moment of the day to listen to the little ones and our body. With the arrival of two children, there is even more need to make room for them. A space that we can already create in pregnancy, even if only mental or emotional, becomes the preamble for the physical and temporal space that we will need later. Then provide for the presence of people who, after birth, can help carry out the most practical tasks – cooking, cleaning, ironing – and competent help to support breastfeeding.

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