title: Twin pregnancy and the risk of preeclampsia: bigger placenta or relative ischemia?
abstract: OBJECTIVE
  Twin pregnancies are a risk factor for preeclampsia with a reported incidence of 2-3 times higher than singleton pregnancies. Soluble fms-like tyrosine kinase 1 (sFlt1), which is a circulating antiangiogenic molecule of placental origin, plays a central role in preeclampsia by antagonizing placental growth factor (PlGF) and vascular endothelial growth factor signaling in the maternal vasculature. Increased sFlt1 and the ratio sFlt1/free PlGF have been shown to antedate clinical signs in preeclampsia. Although the cause of the upregulated sFlt1 in preeclampsia still is not understood clearly, placental ischemia with accompanying hypoxia is thought to play an important role. We therefore hypothesized that the higher risk of preeclampsia in twin pregnancies results from high sFlt1 (or sFlt1/PlGF) and that the sFlt1 upregulation was due to either relative placental hypoxia and/or increased placental mass.
  
  
  METHODS
  Maternal serum samples and placentas from third-trimester twin and singleton pregnancies without preeclampsia were used. Serum samples were analyzed for levels of sFlt1 and free PlGF by enzyme-linked immunosorbent assay and reported as means (in nanograms per milliliter and picograms per milliliter, respectively). Placentas were weighed and examined for content of sFlt1 and PlGF messenger RNA (mRNA) by quantitative polymerase chain reaction and hypoxia inducible factor-1alpha (HIF-1alpha) protein by Western blot.
  
  
  RESULTS
  Soluble Flt1 concentrations in twin pregnancy maternal serum were 2.2 times higher than those that were measured in singleton pregnancy maternal serum samples (30.98 +/- 9.78 ng/mL vs 14.14 +/- 9.35 ng/mL, respectively; P = .001). Free PlGF concentrations were not significantly different between twin and singleton maternal serum samples, but the mean sFlt1/PlGF ratio of twin pregnancy maternal serum samples was 2.2 times higher than the equivalent ratio in singleton pregnancy samples (197.58 +/- 126.86 ng/mL vs 89.91 +/- 70.63 ng/mL, respectively; P = .029). Quantitative polymerase chain reaction for sFlt1 and PlGF mRNA revealed no significant differences between the 2 study groups. Western blot analysis of placental samples for HIF-1alpha revealed a mean ratio HIF-1alpha/actin of 0.53 vs 0.87, for the twins vs singletons placental samples respectively (twins showed lower HIF-1alpha, not higher). The mean weights of twin and singleton placentas were 1246 vs 716 g, respectively (P < .001). Importantly, the placental weights correlated very well with the circulating sFlt1 levels (R(2) = .75).
  
  
  CONCLUSIONS
  In twin pregnancies, circulating sFlt1 levels and sFlt1/PlGF ratios were twice as high as those in singleton pregnancies. The increased serum sFlt1 levels in twin pregnancies were not accompanied by any changes in the levels of sFlt1 mRNA and HIF-1alpha protein in the twin placentas but were correlated with increased placental weight. These findings suggest that the increased risk of preeclampsia in twin pregnancies may be due to increased placental mass that leads to increased circulating levels of sFlt1.
authors.affiliation: Department of Obstetrics & Gynecology, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA 02115, USA.
authors.forename: Yuval
authors.initials: Y
authors.lastname: Bdolah
authors.forename: Chun
authors.initials: C
authors.lastname: Lam
authors.forename: Augustine
authors.initials: A
authors.lastname: Rajakumar
authors.forename: Venkatesha
authors.initials: V
authors.lastname: Shivalingappa
authors.forename: Walter
authors.initials: W
authors.lastname: Mutter
authors.forename: Benjamin P
authors.initials: BP
authors.lastname: Sachs
authors.forename: Kee Hak
authors.initials: KH
authors.lastname: Lim
authors.forename: Tali
authors.initials: T
authors.lastname: Bdolah-Abram
authors.forename: Franklin H
authors.initials: FH
authors.lastname: Epstein
authors.forename: S Ananth
authors.initials: SA
authors.lastname: Karumanchi
mesh_terms: D000328:Adult; D004200:Diseases in Twins; D004797:Enzyme-Linked Immunosorbent Assay; D005260:Female; D015870:Gene Expression; D006801:Humans; D015994:Incidence; D007511:Ischemia / Q000097:blood / Q000503:physiopathology*; D009389:Neovascularization, Pathologic / Q000097:blood; D010920:Placenta / Q000098:blood supply / Q000473:pathology*; D000072483:Placenta Growth Factor; D016133:Polymerase Chain Reaction; D011225:Pre-Eclampsia / Q000453:epidemiology* / Q000209:etiology / Q000503:physiopathology; D011247:Pregnancy; D011257:Pregnancy Proteins / Q000097:blood*; D051496:Receptor, Fibroblast Growth Factor, Type 1 / Q000097:blood*; D012307:Risk Factors; D014427:Twins; D015854:Up-Regulation; D042461:Vascular Endothelial Growth Factor A / Q000097:blood*
chemical_list: C000608036:PGF protein, human; D011257:Pregnancy Proteins; C467484:VEGFA protein, human; D042461:Vascular Endothelial Growth Factor A; D000072483:Placenta Growth Factor; D051496:Receptor, Fibroblast Growth Factor, Type 1