Aim: We examined the heart failure biomarker mid-regional pro-atrial natriuretic peptide during the first trimester of pregnancy in relation to early-onset preeclampsia <34 weeks. Materials and Methods: This case–control study included 34 women with singleton pregnancies with a preeclampsia diagnosis and delivery before 34 weeks of gestation who had attended the routine first-trimester ultrasound scan at 11–13+6 weeks of gestation between August 2010 and October 2015 at the Copenhagen University Hospital Rigshospitalet, Denmark, and 91 uncomplicated singleton pregnancies matched by time of the routine first-trimester blood sampling at 8–13+6 weeks. Descriptive statistical analyses were performed for maternal characteristics and obstetric and medical history for the case versus the control group. Concentrations of mid-regional pro-atrial natriuretic peptide, placental growth factor, soluble fms-like tyrosine kinase-1, and pregnancy-associated plasma protein A between early-onset preeclampsia cases and the control group were compared using Students t-test and the Mann–Whitney U test. Biochemical marker concentrations were converted into multiples of the expected median values after adjustment for gestational age. Results: Mid-regional pro-atrial natriuretic peptide levels were not significantly different between early-onset preeclampsia cases and the control group in the first trimester of pregnancy. As expected, both placental growth factor and pregnancy-associated plasma protein A levels were significantly lower in early-onset preeclampsia, whereas soluble fms-like tyrosine kinase-1 levels were not statistically significantly different. Conclusion: The maternal first-trimester concentration of mid-regional pro-atrial natriuretic peptide, a peptide with multiple biological functions including a relation to cardiovascular disease, was not significantly different in women with early-onset preeclampsia.