Hyperemesis gravidarum (HG) is a pregnancy condition characterised by severe nausea and vomiting during early pregnancy. The experience of HG is for many women a traumatic event. Few studies have investigated a possible association between HG and birth-related posttraumatic stress. The objective of the current study was to assess whether HG increases the risk of birth-related posttraumatic stress symptoms (PTSS). This was a population-based pregnancy cohort study using data from the Akershus Birth Cohort Study (ABC study). A linear mixed model was used to estimate the association between the degree of nausea (no nausea (n = 574), mild nausea (n = 813), severe nausea (n = 522) and HG (hospitalised due to nausea, n = 20)) and PTSS score at 8 weeks and 2 years after birth. At 8 weeks postpartum, women with HG had higher PTSS scores compared to women with no nausea (p = 0.008), women with mild nausea (p = 0.019) and women with severe nausea (p = 0.027). After 2 years, women with HG had higher PTSS scores compared to women with no nausea (p = 0.038). Women with HG had higher PTSS scores following childbirth compared to women with less pronounced symptoms or no nausea at all. After 2 years, women with HG still had higher PTSS scores compared to women with no nausea. Although the overall differences in PTSS scores were small, the results may still be of clinical relevance.