Perineal injury is the most common complication of vaginal delivery, with an incidence of up to 90%, among which obstetric anal sphincter injury (OASI) is a serious type, which can lead to long-term sequelae such as pelvic floor dysfunction and anal incontinence, which seriously affects the quality of maternal life. There has been a lack of clear evidence on the association between gestational weight gain (GWG) and perineal injury in primiparous women with underweight preconception (BMI<18.5) due to their special physical underlying conditions. A retrospective cohort study published in the International Journal of Obstetrics and Gynecology (Int J Gynecol Obstet) in December 2025 subverted the traditional perception that "weight gain during pregnancy needs to be strictly met to meet standards to prevent damage", and at the same time identified three key high-risk factors, providing an accurate evidence-based basis for clinical management.
Research the relevant situation
1. Crowds and groups
The study included 4,034 primiparous women with a pre-pregnancy BMI of <18.5 in a tertiary medical center in Israel from 2012 to 2023, excluding preterm birth, diabetes, multiple pregnancies, etc. According to the National Academy of Medicine (IOM) guidelines, they were divided into three groups according to gestational weight gain:
❖ Group A (1976 people, 49.0%): insufficient weight gain (<12.5kg);
❖ Group B (1575 people, 39.0%): weight gain up to standard (12.5-18kg);
❖ Group C (483 people, 12.0%): excessive weight gain (>18kg).
2. Outcome indicators
❖Primary outcomes: Overall incidence of perineal injury, including spontaneous perineal tears, labia tears, and episiotomy at all levels;
❖ Secondary outcomes: obstetric anal sphincter injury (OASI, including 3-4 degree perineal tears).
3. Analysis method
Multivariate logistic regression model was used to adjust for potential confounding factors such as maternal age, smoking history, delivery mode, and neonatal weight, and the association between pregnancy weight gain and various factors and perineal injury was analyzed.
Findings
1. Disruptive finding: Weight gain during pregnancy is not associated with perineal damage
❖ Overall incidence of perineal injury: 82.6%, 82.0% and 83.4% in the three groups, respectively, with no statistical difference between the groups (P=0.739);
❖ Breakdown of injury types: There was no significant difference in the incidence of spontaneous perineal tear (49.6%-53.2%), episiotomy (34.8%-37.4%), and labia tear (10.0%-10.8%) between the three groups
❖ The incidence of OASI was 1.0%-1.1% in all three groups, which was completely consistent (P=0.991), suggesting that the risk of severe perineal injury did not change regardless of whether the weight gain was insufficient, standardized or excessive
2. Three high-risk factors: the "culprit" of perineal injury
(1) Vacuum-assisted delivery (highest risk)
❖ The risk of perineal injury directly soared by 3.88 times (OR = 3.876, 95% CI 2.741-5.481, P<0.001);
❖ It is an independent high-risk factor for OASI, with a risk increase of 2.19 times (OR=2.190, 95% CI 1.136-4.222, P=0.019), and the clinical indications for use need to be strictly controlled.
(2) Epidural anesthesia
❖ The risk of perineal injury increased by 51.9% after use (OR=1.519, 95% CI 1.237-1.865, P<0.001);
❖ It may be related to the weakening of maternal perception during childbirth after anesthesia, improper force method, or prolongation of the second stage of labor (in this study, the prolongation of the second stage of labor has an increased risk of injury, P=0.051).
(3) High body weight of newborns
❖ For every 1 kg increase in neonatal body weight, the risk of perineal injury increased by 2.12 times (OR=2.124, 95% CI 1.666-2.707, P<0.001);
❖ The impact on OASI was more significant, with the risk of OASI soaring 3.24 times for every 1 kg of body weight gain (OR=3.237, 95% CI 1.450-7.226, P=0.004), which is a key indicator for predicting severe tears.
3. Potential protective factors (not recommended intervention, only for research findings)
❖ Pre-pregnancy BMI: The closer the BMI is to 18.5, the lower the risk of perineal injury (OR=0.855, 95% CI 0.756-0.967, P=0.012), suggesting that the pelvic floor tissue conditions of women with better basal weight may be better;
❖ Smoking: Smoking during pregnancy reduces the risk of perineal injury in women (OR=0.569, P=0.001), but smoking can lead to a variety of adverse maternal and infant outcomes such as preterm birth and low birth weight, which must not be used as a protective measure, and its mechanism needs to be further studied.
Clinical enlightenment
1. Weight management: No need to "stumble" to reach the standard
❖No need to worry about weight gain values: Insufficient or excessive weight gain during pregnancy will not increase the risk of perineal injury, and clinical practice should focus on adequate maternal nutrition and normal fetal development, rather than mechanically pursuing a weight gain range of 12.5-18kg;
❖Individualized nutritional support: Women who are underweight before pregnancy are prone to anemia and other nutritional deficiencies, and need to supplement sufficient protein and vitamins to avoid affecting the elasticity of pelvic floor tissues due to malnutrition, but there is no need to deliberately gain weight to prevent damage
2. Risk prevention and control: Focus on 3 core targets
(1) Strictly control vacuum-assisted delivery: For women who are underweight before pregnancy, priority should be given to natural childbirth, and vacuum assistance should only be used under clear indications such as fetal distress and labor stagnation, and the strength and angle should be standardized during operation;
(2) Optimize epidural anesthesia management: After use, it is necessary to guide the mother to exert force one-on-one, avoid the second stage of labor for more than 3 hours, and evaluate the progress of labor in time;
(3) Predict fetal weight: Accurately estimate the weight by ultrasound before delivery, and for fetuses ≥ 3.5kg, take both hands perineal protection to control the delivery speed of the fetal head during delivery, and perform a midlateral shear episiotomy if necessary
3. Personalized care: targeted and enhanced protection
❖Pre-pregnancy pelvic floor assessment and training: Routinely assess pelvic floor muscle strength during pregnancy, and carry out Kegel exercises for weak mothers to enhance tissue toughness;
❖ Multidisciplinary coordination: obstetricians, midwives, and anesthesiologists jointly formulate delivery plans, dynamically assess high-risk factors throughout the process, and adjust interventions in a timely manner;
❖ Avoid misleading interventions: It is not recommended to allow mothers to gain excessive weight for "injury prevention", focusing on intrapartum operating norms
Study limitations
❖ Single-center retrospective design, data reliance on medical records, information bias, and the study population is mainly Caucasian race, so generality needs to be cautious;
❖ Pelvic floor muscle thickness, collagen level and other indicators were not collected, and the injury mechanism could not be further analyzed.
❖ There is a lack of smoking dose-response relationship analysis, and its "protective effect" needs to be verified by prospective studies.
Primiparous women who are underweight before pregnancy do not need to be strictly limited in weight gain during pregnancy, and the risk of perineal injury is not related to the amount of weight gain. Clinically, we should focus on preventing and controlling the three high-risk factors of vacuum-assisted delivery, epidural anesthesia use, and high newborn weight, and minimize the risk of injury by standardizing intrapartum operations, individualized management, and preconception preparation. This finding fills the research gap in people with special weight and provides accurate evidence-based support for the management of childbirth in primiparous women with low preconception weight.
责编：Lucy