This is an observational study in 3 cohorts of monochorionic diamniotic (MCDA) twins. The aim of the study is to assess whether unequal placental sharing can be predicted in pregnancy and whether it is predictive of pregnancy outcome.

In all cohorts, serial ultrasound will be performed during pregnancy. MRI will be offered once during pregnancy. After delivery, placental examination will be performed.

In unselected MCDA twins, the investigators will assess the relationship between unequal placental sharing and the risk of fetal or neonatal loss. The investigators will also establish whether umbilical vein measurements or MRI can be used to predict placental sharing.

In cases complicated by twin-to-twin transfusion syndrome (TTTS), the investigators will look at the association between unequal placental sharing and intrauterine fetal demise after laser surgery.

In cases complicated by selective intra-uterine growth restriction (sIUGR), the the relationship between unequal placental sharing and the risk of fetal or neonatal loss and birth prior to 34 weeks will be assessed.

Furthermore, data on neurological outcome at 2 years of age in all cohorts will be collected.

Inclusion Criteria:

• MCDA cohort

  • Ongoing monochorionic diamniotic twin pregnancy between 11 and 14 weeks, as determined by the crown rump length of the larger twin in spontaneous conceptions and by the date of insemination or embryonic age at replacement in pregnancies resulting from subfertility treatment
  • Women aged 18 years or more, who are able to consent
  • Written informed consent to participate in this study, forms being approved by the Ethical Committees

• TTTS cohort

  • Monochorionic diamniotic twin pregnancy, with gestational age defined in the first trimester as mentioned above
  • Complicated by TTTS according to the definition used in the Quintero staging of TTTS (DVP of donor < 2 cm and DVP of receptor > 8 cm before 20 weeks or > 10 cm after 20 weeks
  • Undergoing laser surgery for TTTS
  • Women aged 18 years or more, who are able to consent
  • Written informed consent to participate in this study, forms being approved by the Ethical Committees

• sIUGR cohort

  • Monochorionic diamniotic twin pregnancy, with gestational age defined in the first trimester as mentioned above
  • Complicated by sIUGR defined as a discordance in EFW of ≥ 20%, with the percentage of discordance being calculated as (EFW larger twin - EFW smaller twin)/EFW larger twin x 100.
  • Women aged 18 years or more, who are able to consent
  • Written informed consent to participate in this study, forms being approved by the Ethical Committees

Exclusion Criteria:

• Age < 18 years
• Inability to consent

The study population exists of women with a monochorionic twin pregnancy. Three cohorts will be created: a cohort of uncomplicated MCDA twins (included at the first trimester scan), a cohort of MCDA twins complicated by TTTS and undergoing laser surgery (included at the time of diagnosis) and a cohort of MCDA twins complicated by selective growth restriction (included at the time of diagnosis).