**4.2. Monochorionic pregnancies**

It is a well-established fact that surveillance in monochorionic pregnancies has to be closer in relation to a dichorionic pregnancy. Finberg et al. [46] recommended ultrasound monitoring for noncomplicated monochorionic twins every 3–4 weeks. As the pattern mentioned previously, nowadays, routine scans are performed more frequently: they are performed every 2–3 weeks, starting from the gestational age of 16 weeks. Finally, in some cases, surveillance is even closer: a follow-up scan can be repeated every 2 weeks.

The parameters that are necessary to be evaluated in these follow-up scans are estimated fetal weight and fetal biometry, amniotic fluid volume, and Doppler assessment of the umbilical artery [49].
