TWIN PREGNANCY (or multiple pregnancy) is a reproductive phenomena of great interest, especially because of its maternal and fetal problems entails. Twin pregnancy frequency can be estimated at about 2-3 cases per 100 pregnancies with a higher prevalence in African populations and lowest in Asians. Increase recorded in Western countries in the last decades is correlated to the introduction of modern techniques of artificial insemination.
There are two twin kinds: those DIZYGOTIC (more frequent, about 2/3 of the total) in which it occurred a fertilization of two different oocytes from as many sperm and therefore are not identical and those MONOZYGOTIC (more rare, about 1 / 3 of the total) in which a single oocyte after fertilization, by chance, it undergoes a precocious division giving rise to two completely identical embryos.
INCIDENCE of dizygotic twins is therefore particularly linked to multiple ovulations and is influenced by predisposing genetic factors that instead does not seem present in homozygous twins. Generally, pregnancies originating from two different oocytes lead to separate placentas and amniotic sacs for each fetus, while in monozygotic exist a placenta and an amniotic sac for each fetus (bicorial and biamniotic monozygotic twin pregnancy); if the division of the zygote occurs in the first 3 or 4 days after conception (28% of cases), there will be a common placenta and two separate amniotic sacs (biamniotic monochorionic monozygotic twin pregnancy); if the division is between the 5th and 8th day after conception (70% of cases), there will be a placenta and a single amniotic sac (monoamniotic monochorionic monozygotic twin pregnancy) when division takes place after the 8th day after conception (2% of cases).
See Table 1.
|TABLE 1: Summary illustration of how one or more oocytes and sperm matching and on how time point of zygote split will later configure twin pregnancy kinds.|
|2 sperm and 2 oocytes (dizygotic pregnancy)||1 sperm and 1 oocyte (monozygotic pregnancy)|
|Split between 0 and 4 days||Between 5 and 8 days||Between 9 and 12 days|
|twin bicorial biamniotic pregnancy (2 separate placentas and 2 separate amnion)||Twin bicorial biamniotic pregnancy||Twin monochorionic biamniotic pregnancy||Twin monoamniotic pregnancymonochorionic|
In monozygotic pregnancies, the later takes the one fertilized oocyte splitting and the higher is the risk of having conjoined twins, also known as Siamese.
Twin pregnancy should always be considered a “high-risk pregnancy” and for this reason deserving a special healthcare pathways. In this regard considering that one twin has a risk of dying or having a permanent damage about 5-10 times higher than other fetuses and that the multiple pregnancy also increases the maternal risk during both gestation and birth.
In the twin pregnancy obstetrician general evaluation of the risk is particularly important the CHORIONICITY, that is the number of placentas that becomes a discriminating factor in the clinical management. While in bicorial biamniotic pregnancies (2 placentas) the risk of fetal mortality is 9%, in monochorionic (one placenta) biamniotic pregnancies rises to 25% and over 50% in monochorionic monoamniotic pregnancies (fetuses are contained in a single amniotic sac).
In summary, in twin pregnancy chorionicity corresponds to the number of placentas (one or more), the amnioticity to the number of amniotic sacs (1 or more). Another important element of risk is the number of fetuses that is related in direct proportion with the maternal-fetal morbidity and mortality.
Now I want to analyze what always distinguishes the twin pregnancy to a singleton pregnancy from the point of view of the MATERNAL-FETAL ISSUES:
– The chance of miscarriage is increased;
– It increased the chance of death in utero and, more generally, the peri-natal mortality, especially in monochorionic pregnancies, estimated to be around 10%, and in those monochorionic – monoamniotic that reaches 40-50%;
– Fetal congenital malformations incidence increases both in terms of structural and chromosomal defects (in monochorionic twins is approximately 1.5 times);
– Frequently, fetal growth reduction and fetus-fetal transfusion chance in monochorionic twins (Twin to Twin Transfusion Syndrome – TTTS), can be acute or chronic and often, unfortunately, with dramatic effects;
– In twin pregnancies (multiple) autonomic symptoms (nausea, vomiting), weight gain, fatigue, eating disorders, respiratory disorders, abdominal pain and pubalgia are more frequent;
– Some maternal obstetrical pathologies are unfortunately much more frequent and among these we can include anemia, premature labor, premature rupture of membranes, gestational diabetes and hypertension and post-partum haemorrhage.
We have mentioned the frequent occurrence of premature birth: a twin pregnancy normally ends 3-4 weeks before the 40th week. The gestional age is, however, also strongly influenced by the number of fetuses.
MULTIPLE PREGNANCIES are a serious welfare issue. Although most of the Reproductive Medicine Centers try to avoid their incidence, they are now linked not only to in vitro fertilization attempts, but rather to drug therapy for induction of ovulation. The fetal and maternal risks – even in terms of mortality – are so high that many centers offer the selective reduction of the number of embryos, which are generally brought to 2.
Twin pregnancy CLINICAL MONITORING requires a more frequent number of clinical examinations (obstetricians but also of internal medicine and cardio-respiratory), instrumental examinations (mainly ultrasound examinations even of second level with Doppler flow and Cervicometry) and blood tests.
A particular attention should also be paid to food. It requires to be suited to particular nutritional needs and to the screening of fetal malformations. CHILDBIRTH is undoubtedly a moment of criticality both in terms of Birth Time (in some cases at higher maternal or fetal risk and it is recommended an early birth to 32-34 weeks) and in terms of modality (vaginal or by C-Section).
In many institutes vaginal childbirth is generally allowed when local maternal conditions (cervix and birth canal) are favorable, that is fetuses are of good weight and in good health, both in cephalic presentation, while others allow the childbirth even when the second twin is in breech presentation. There are no definitive data on this issue.
Vaginal childbirth general contraindications: a multiple pregnancy and that monoamniotic, the first fetus in breech presentation, fetal pathologies.
In an environment where will be performed a twins vaginal birth should be always present an obstetrical and neonatology EXPERT TEAM, the operating theatre should be available immediately as well as an Anaesthetist and a Blood Transfusion Centre. It ‘also important the availability of a twin Continuous Cardiotocography and an ultrasound device (the second twin could change presentation).
The discussion on the method of childbirth should always be addressed earlier, usually between the 32nd and the 34th week in monochorionic pregnancies and between the 34th and the 36th week in diamniotic.
I would like to point out a very important aspect for a good reproductive success in these complicated pregnancies: INFORMATION. Women with twin pregnancy should “know” the characteristics of their particular gestation, the risks and the need for a separate clinical management.