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It is yet unknown how similar women's hormone levels are during successive pregnancies, and very little is known about the degree to which siblings experience similar prenatal environments. Given the importance of understanding how women's reproductive life histories exert cumulative effects on health via hormone exposure, and the importance of understanding how fetal programming via endocrine signaling affects sibling trait concordance, here, we address this important lacuna in the literature.This study aimed to investigate how consistent women's hormone profiles are across two successive pregnancies.This longitudinal, prospective study followed a cohort of 28 women across two pregnancies (PREG 1 and PREG 2). Women's circulating hormone levels were assessed from blood samples at 25, 31, and 37 weeks' gestation for adrenocorticotropic hormone (ACTH), placental corticotropin-releasing hormone (pCRH), cortisol, estradiol, and progesterone. ACTH and cortisol levels were assessed 3 months postpartum. Research questions include: Are hormone levels in PREG 2 significantly different from levels in PREG 1? What proportion of variance in PREG 2 hormone levels is attributable to variance in PREG 1 levels? Are hormone levels more stable between PREG 1 and PREG 2 compared with postpartum phases following these pregnancies? Is pCRH, which is completely placentally derived, less similar than other hormones across successive pregnancies?Pregnant women attended study visits at a university psychobiology laboratory in Southern California.Comparisons of hormone concentrations across women's successive pregnancies via paired t test revealed substantial consistency from one pregnancy to another, with only significant differences between pregnancies for pCRH. Regressions revealed substantial predictability from one pregnancy to another, with between 17-56% of PREG 2 variances accounted for by PREG 1 values. Women exhibited lower degrees of consistency and predictability in hormone levels across postpartum phases compared with gestational concentrations. This is the first study to describe maternal and placental hormone levels across successive pregnancies.