Pregnancy complications, such as preeclampsia (PEc), have an increased incidence among patients with type 1 diabetes (T1DM), possibly because of maternal vascular involvement. The prototypic long pentraxin, pentraxin 3 (PTX3) is an acute phase reactant critically associated with vascular injury. PTX3 concentrations selectively increase in pregnant women with PEc. Here, we measured PTX3 levels in 37 consecutive pregnant patients with T1DM. Compared with PTX3 levels in healthy pregnant women at identical gestational ages, PTX3 was significantly elevated in pregnant women with diabetes. Patients with pre-existing nephropathy, a well-characterized microvascular complication of diabetes, have even higher PTX3 concentrations and worse maternal and fetal outcomes. Six/thirty-four diabetic non-nephropatic patients developed PEc: PTX3 levels rose abruptly weeks before PEc manifested (p = 0.0375). PTX3 may represent a valuable marker for early detection and prediction of PEc in patients with T1DM.