Risk of Hypertensive Disorders in Pregnancy After Fresh and Frozen Embryo Transfer in Assisted Reproduction: A Population-Based Cohort Study With Within-Sibship Analysis

Hypertension. 2023 Feb;80(2):e6-e16. doi: 10.1161/HYPERTENSIONAHA.122.19689. Epub 2022 Sep 26.


Background: Frozen embryo transfer (frozen-ET) is increasingly common because of improved cryopreservation methods and elective freezing of all embryos. Frozen-ET is associated with higher risk of hypertensive disorders in pregnancy than both natural conception and fresh embryo transfer (fresh-ET), but whether this is attributable to parental factors or treatment is unknown.

Methods: Using the Medical Birth Registries of Denmark (1994-2014), Norway, and Sweden (1988-2015), linked to data from national quality registries and databases on assisted reproduction, we designed a population-based cohort study with within-sibship comparison. We included 4 426 691 naturally conceived, 78 300 fresh-ET, and 18 037 frozen-ET singleton pregnancies, of which 33 209 sibships were conceived using different conception methods. Adjusted odds ratios (aOR) of hypertensive disorders in pregnancy for fresh-ET and frozen-ET versus natural conception with 95% CI were estimated using multilevel logistic regression, where random effects provided conventional population-level estimates and fixed effects gave within-sibship estimates. Main models included adjustment for birth year, maternal age, parity, and country.

Results: Risk of hypertensive disorders in pregnancy was higher after frozen-ET compared to natural conception, both at population-level (7.4% versus 4.3%, aOR, 1.74 [95% CI, 1.61-1.89]) and within sibships (aOR, 2.02 [95% CI, 1.72-2.39]). For fresh-ET, risk was similar to natural conception, both at population-level (aOR, 1.02 [95% CI, 0.98-1.07]) and within sibships (aOR, 0.99 [95% CI, 0.89-1.09]).

Conclusions: Frozen-ET was associated with substantially higher risk of hypertensive disorders in pregnancy, even after accounting for shared parental factors within sibships.

Keywords: cryopreservation; embryo transfer; fertilization in vitro; hypertension; pre-eclampsia; pregnancy; siblings.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Cohort Studies
  • Embryo Transfer / adverse effects
  • Embryo Transfer / methods
  • Female
  • Fertilization in Vitro / adverse effects
  • Fertilization in Vitro / methods
  • Humans
  • Hypertension, Pregnancy-Induced* / epidemiology
  • Pregnancy
  • Reproduction
  • Retrospective Studies