**2. Diagnosis**

Imaging of the chorion from 5 to 6 weeks after LMP (Last Menstrual period). Imaging of the fetal poles from the 7th week after LMP. Fetal heartbeat from the 7th week after LMP.

and in transvaginal ultrasound we have: Imaging of the chorion from the end of the 4th week after LMP. Imaging of the amniotic sacs from the 5 'week after LMP. Imaging of fetuses from the end of the 6th week after LMP. Fetal heart rate from the 6th week after LMP.

Ultrasound monitoring of a twin pregnancy has two main objectives: to determine the chorionicity and zygosity of the pregnancy due to the higher risk of conceiving a chromosomally abnormal fetus compared to single pregnancies, the risk of twin-twin transfusion syndrome and intrauterine growth restriction.

In particular, the determination of chorionicity that is easier between 11 and 13 weeks (identification of placental points L and T, number of amniotic sacs) is a prerequisite for monitoring twin pregnancies, early detection FFT syndrome, screening for congenital abnormalities. It is often difficult to diagnose monoamniotic twins because it is difficult to visualize them due to the small thickness of the membrane or in cases of oligamnion. The existence of entangled umbilical cords confirms the existence of a single amniotic cavity. The estimation of the thickness of the diaphragm has 83% prognostic value for dichorionic pregnancy, in cases of thin membrane 83% prognostic value for monochorionic diamniotic pregnancy. In dichorionic pregnancies, two placentas are visualized but in monochorionic pregnancies when two layers are detected during the examination of the diaphragm, then these are monochrorionic diamniotic pregnancies, if they are four layers then these are dichorionic diamniotic pregnancies. Ultrasound examination in twins with a single placental mass in the presence of a triangular protrusion of the placental tissue, a peak sign of a twin pregnancy or a lambda or a double apex indicates the existence of two placentas therefore a dichorionic, diamniotic pregnancy. In cases of non-detection of the lambda point but certification of the T sign at the membrane entrance, it indicates monochorionic twin pregnancies. In monochorionic placentas the selective discontinuation of one twin leads to an increase in thromboplastin and poses a risk of amniotic fluid embolism for both the other fetus and the pregnant mother. Fetuses of different genders are always dichorionic and diamniotic. The initial measurements of twin fetuses based on Crown-rump length (CRL) and nuchal translucency do not differ from those of single pregnancies. Amniocentesis is the method of choice in many cases. The rate of spontaneous abortion after amniocentesis does not differ in twin pregnancies achieved by assisted reproduction methods compared to those achieved by spontaneous conception. In pregnancies achieved through assisted reproduction technology, there are high levels of β-HCG with an increased rate of premature maturation and also an increased rate of placental abnormalities, as well as a vaginal spotting. Twin pregnancies have a higher rate of aneuploidy [22–30].

#### **2.1 Monitoring of multiple pregnancy**

Chorionicity should be determined on the first ultrasound at the first visit. Possible prenatal screening problems should be highlighted, several visits are necessary for intensive monitoring of both the pregnant mother and the fetus.

#### *2.1.1 Frequency of visits*

#### *2.1.1.1 Monochorionic/diamniotic twins*

Every 2 weeks until the 16th–24th week of pregnancy in order to exclude TTTS. Especially, during 18th week of pregnancy it should be performed for a detailed

#### *Twin Pregnancies Labour Modus and Timing DOI: http://dx.doi.org/10.5772/intechopen.95982*

examination of fetal development and during 22–24 weeks of pregnancy for a detailed fetal heart ultrasound. After 24th week, until 32nd week of pregnancy, it should also be performed every 2 weeks, and after 32nd week of pregnancy it should be performed every week until the time of delivery, in order to evaluate in time the possible pathology of the intrauterine fetal development.
