Here we show that among a well-characterized cohort of healthy, low-risk singleton pregnant women at term, more than half do not achieve optimal weight gain during pregnancy and that such abnormal GWG associated with higher odds of adverse childbirth and perinatal outcomes. Specifically, women with iGWG were more likely to give birth in the early term time-window and to an SGA neonate. In turn, women who gained more weight than recommended more commonly experienced prolonged pregnancy, CS in labor, high degree perineal tears, PPH ≥ 1000 mL, and LGA newborns.
Although the importance of appropriate GWG is well established worldwide, many women still gain either to little or too much weight during gestation.
A recent systematic review and meta-analysis on the topic has reported gain outside NAM ranges for 70% of pregnant women, with 23% gaining below and 47% above . Similarly, in our cohort we observed 52.3% of women not achieving adequate GWG. However, insufficient weight gain occurred more frequently than excessive gain (33.5% versus 18.8%). This finding could be likely explained by the characteristics of our study population, including only healthy women without comorbidities and a normal pBMI, and it is in line with previously reported data suggesting higher odds of insufficient and excessive gain among normal weight and overweight/obese women, respectively [7, 30].
Women with iGWG in our cohort showed higher rates of early term birth, whereas excessive weight gain associated with increased likelihood of late and post term birth.
The timely onset of labor and delivery is an important determinant of perinatal outcome. Early term births between 37- and 38-weeks’ gestation have been shown to associate with higher neonatal and infant morbidity and mortality compared to full term births.  Similarly, increased perinatal morbidity and mortality have been identified as pregnancy extends beyond 41 weeks, thus requiring a more intense antenatal surveillance and use of resources . We did not observe cases of stillbirth or early neonatal death. However, women with eGWG and prolonged pregnancy ≥ 41 weeks underwent increased antenatal surveillance for fetal wellbeing assessment according to our institutional protocol. 
We observed unfavorable outcomes related to altered fetal growth of weight gain outside NAM recommendations. Specifically, women gaining insufficient weight gave more commonly birth to an SGA neonate, whereas eGWG women to an LGA newborn. These findings are consistent with previous reports, showing elevated rates of SGA in women with normal pBMI and iGWG, as well as increased odds of LGA for excessive gain across all pBMI categories. [3, 34–36] Considering the potential short- and long-term complications of SGA and LGA, [37–39] prevention of both conditions by promoting adequate GWG might be advisable from an obstetric and public health point of view.
Regarding additional relevant childbirth outcomes, low-risk women who gained more weight than recommended more commonly experienced CS in labor, high degree perineal tears, and PPH ≥ 1000 mL.
While the finding regarding CS is in line with previous works, [3, 40] increased odds of high degree perineal tears and PPH have never been reported before in such a well characterized low-risk obstetric population.
We identified only two studies on genital tract trauma among healthy, low-risk pregnant women with midwife-led care; however, both included overweight and obese women and did not adjust analyses for different pBMI values and LGA rates, [16, 17] known risk factors for perineal trauma. By assessing only healthy women with a normal pBMI and no comorbidities, we did not incur in potential biases related to maternal pregravid weight status and its strong independent effect on perinatal outcomes.  Although the overall rate of genital tract trauma was low in our cohort (1.2%), women with eGWG displayed a two-fold risk increase compared to their aGWG counterparts.
As per PPH, we could not find any research work on this outcome in healthy, uncomplicated pregnant women with abnormal GWG. Considering that PPH is a leading cause of maternal morbidity and mortality,  our finding of a 1.5-fold increase in the odds of PPH deserves further attention. If confirmed in larger prospective studies, consideration could be given to inclusion of eGWG in PPH risk assessment tools,  to increase preparedness in this otherwise low-risk population.
A major strength of this study is that yields new evidence on GWG according to the 2009 NAM recommendations in a large and well-characterized population of healthy, low-risk uncomplicated singleton pregnant women with normal pBMI.
Additional strengths are as follows. Prepregnancy weight and height were recorded in early pregnancy and therefore prospective regarding outcome, thus avoiding recall bias. pBMI was ascertained by using self-reported preconception weight, which has been shown to be more accurate for assessing total GWG as compared to first trimester weight, particularly among women with normal pBMI, [42, 43] and it is recommended by the 2009 NAM guidelines.  Also, we excluded women with a > 2 Kg difference between self-reported weight and weight measured at the first antenatal visit < 14 weeks, thus further limiting errors in pBMI categorization. [4, 19, 44] Biases in calculating total GWG were limited by measuring last maternal weight upon admission for childbirth. Institutional protocols to guide supervision of low-risk pregnant women during pregnancy and childbirth are consistently followed at our center, and, as such, a standardized care model is ensured. Finally, consistent definitions of the outcomes of interest, which included a wide range of childbirth and perinatal outcomes, were applied, thus increasing clinical usefulness and comparability.
Our study has some limitations.
First, it was a single-center study, which limit generalizability of our findings and lead to the need of external validation. Second, the retrospective design may have conducted to unrecognized confounding factors. Last, we did not assess the potential impact of abnormal GWG on medium-term outcomes, such as weight retention and rate of breastfeeding at 6 months post-partum.