Stress urinary incontinence (SUI) is a common problem in pregnant women. The prevalence is low in the first trimester and increases during pregnancy reaching figures that range between 26.7% and 60.0% in the last weeks (1).  There are two theories which attempt to explain the pathophysiological mechanism of gestational SUI. The first one suggests that increased hormonal concentrations can cause local tissue changes that affect the continence mechanism. The second theory postulates that the mechanical pressure of the enlarging uterus on the bladder and the pelvic floor could be the cause of SUI. On the other hand, obesity is perhaps the most clearly established risk factor for urinary incontinence (UI) in women. Both higher body mass index (BMI) and greater weight gain are associated with increased risk of incident UI. Even more, there is adequate evidence that obesity increases intra-abdominal pressure, predisposing to SUI (2).
The aim of the study was to investigate incident SUI at first trimester (10-12 weeks) and cumulative incidence at the end of pregnancy (37-40 weeks). We also evaluated the association between SUI and maternal weight in both periods.  We hypothesized that increased BMI just before pregnancy might play a role in incident SUI at first trimester, and that pregnant women with greater weight gain throughout pregnancy would have higher risk of suffering from SUI at the end of pregnancy.

This was a prospective study including primigravid women with a singleton pregnancy who were invited to participate in the first weeks of pregnancy, from January to June 2023. Our aim was to study only new cases of SUI, so those women who referred any kind of urinary incontinence before pregnancy were excluded from the study. Other exclusion criteria were: history of malignant diseases or fractures in the pelvic area; urogenital malformations; neurological disorders; pregestational diabetes mellitus; pregnancy loss; and preterm birth. 
Pregnant women recruitment was carried out through the appointment list to perform the 12-week ultrasound. We first contacted the women by phone and explained the nature the study. Afterwards we sent the informed consent and a self-administered questionnaire via email or by postal mail to the women who agreed to participate. Pregnant women were instructed to answer the questionnaire in the first trimester (10-12 weeks) and at the end of pregnancy (37-40 weeks) and return it by the same way. Those women who had not completed both questionnaires during pregnancy had the opportunity to do so during their admission after delivery.
The diagnosis of of SUI was based on symptoms. We used the self-administered questionnaire “Pelvic floor questionnaire for pregnant and postpartum women” which included a question about the symptom of SUI according to the ICS definition (3). Age, height, weight just before pregnancy, weight at the time of completing the questionnaire, and family history of pelvic floor disfunctions (PFD) were collected through items from the same questionnaire. Weight gain during pregnancy was calculated by subtracting the pre-pregnancy weight from the weight of pregnant women at term. The weight increase was categorized into < 10 kg and ≥ 10 kg.
Statistical analysis was performed using the Statistical Package for the Social Sciences (SPSS, version 26.0 for Windows). The potential associations between age, BMI and weight gain during pregnancy with SUI were explored by comparison of means (Student T test, Mann Whitney U test) and percentages (Chi-square and Fisher’s test). Multiple logistic regression was used to investigate independent associations. Statistical significance was set at p=0.05.

During the study period we recruited 260 primigravid women who were continent before pregnancy. Of the total, 208 answered the questionnaire in the first trimester and at the end of pregnancy, forming the study group. Mean age was 33.92 (SD:4.41), mean BMI just before pregnancy was 23.54 (SD: 4.23), mean BMI at the end of pregnancy was 27.35 (SD: 4.37) and mean weight gain was 10,24 (SD: 4.25). Of the total 37 women (17.8%) reported family history of PFD.
At 10-12 weeks of pregnancy, SUI affected 38 (18.3%) pregnant women. Of the total 36 (17.3%) reported losses less than once a week, and 2 (1.0%) reported losses one or more times a week. In the late pregnancy (37-40 weeks) SUI affected 82 (39.4%) women. Of the total 64 (30.8%) reported losses less than once a week, 13 (6.3%) reported losses one or more times a week, and 5 (2.4%) reported daily losses.
Women who reported SUI in the first trimester had a significantly higher BMI just before pregnancy (25.34 ± 5.93 vs. 23.13 ± 3.65; p=0.039). Age was also higher, but the difference did not reach statistical significance (34.97 ± 4.57 vs. 33.69 ± 4.35; p= 0.13).
Pregnant women who reported SUI at the end of pregnancy had a significantly greater weight gain during pregnancy (11.04 ± 4.63 vs. 9.73 ± 3.93; p=0.034). The were no significant differences for age, BMI just before pregnancy and BMI at the end of pregnancy (table 1). 
When we categorized weight gain into < 10 kg and ≥ 10 kg, we observed that women with a weight gain ≥ 10 kg had a higher cumulative incidence of SUI at the end of pregnancy (46.7% vs. 29.1%; p=0.01). We adjusted this analysis with age, BMI at term, and family history of PFD, and observed that women with a weight gain ≥ 10 kg more than doubled the risk of SUI at the end of pregnancy (OR:2.27; 95%CI:1.24-4.13).

SUI appears in 18,2% of primigravid women in the first trimester of pregnancy. Women with a higher BMI before pregnancy are at greater risk. These results reflects that the mechanism of urinary continence is modified from the first weeks of pregnancy leading to SUI, and that these changes have a higher impact on women who are overweight before pregnancy.
SUI increases in pregnant women at term reaching a cumulative incidence of 39.4%. Those women with a weight gain ≥ 10 kg more than doubles the risk of SUI at the end of pregnancy. These results suggest that the continence mechanism progressively weakens throughout pregnancy being more evident in pregnant women with greater weight gain.

According to the mechanical theory, it seems that maternal weight plays an important role in gestational SUI. This effect may be favored by the hormonal changes that occur during this particular period. 
We should advise women to control their weight before pregnancy, and to avoid gaining excessive weight during pregnancy to minimize changes in the continence mechanism leading to gestational SUI.

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D’Ancona CD, Haylen BT, Oelke M, Herschorn S, Abranches-Monteiro L, Arnold EP, Goldman HB, Hamid R, Homma Y, Marcelissen T, Rademakers K, Schizas A, Singla A, Soto I, Tse V, de Wachter S. An International Continence Society (ICS) Report on the Terminology for Adult Male Lower Urinary Tract and Pelvic Floor Symptoms and Dysfunction. Neurourol Urodyn 2019; 38(2):433-477.

Continence 12S (2024) 101396
DOI: 10.1016/j.cont.2024.101396