Table 2 Main Findings of included studies and adjustments

1

Barclay 2020

Normal BMI = Reference group (20 < BMI < 21.99)

Obese (BMI ≥ 30) final parity = ß :-0.682 (SE = 0.058) (CI: -0.796 to -0.568)

Birth cohort, Birth year, Convenience age, Age at time of the conscription, Completed sibling group size, education; attainment education, Cumulative income

2

Frisco 2012

Early adult BMI is negatively and significantly related to ever having a birth among Cohort 1 population.

Each increase in BMI = ↓ Odds of first birth by 5.9% (1-exp (-0.061)).

Early adult BMI is not significantly associated with first birth among Cohort 2 population.

Race/ethnicity, women’s educational background and their mothers’

years of education, women’s age of menarche, fertility expectations

3

He 2018

Obese group compared to normal BMI group (Ref):

Tried for > 12 month to become pregnant: RR:3.899 (CI:1.95–7.77)

Childhood age, follow-up length, highest parental education, and marital status

Obese group compared to normal BMI group (Ref):

Seen a doctor due to trouble become pregnant : RR :2.36 (CI:0.95–5.85)

Obese group compared to normal BMI group (Ref):

Infertility: RR : 2.94 (CI:1.48–5.84)

4

Jacobs 2016

Child BMI (< 9 Years) Obese compared to normal BMI (Ref): Infertility (Tried Unable) : RR = 1.76(CI:1.04–2.97)

Education level, race, tobacco use history, current BMI and adult income

Early teen BMI (9–12 Years) Obese compared to normal BMI (Ref): Infertility (Tried Unable) : RR = 1.94(CI:1.22–3.08)

Late teen BMI (13–18 Years) Obese compared to normal BMI (Ref): Infertility (Tried Unable) : RR = 1.44(CI:1.03–2.03)

5

Joleka 2008

Obese women compared to normal weight (Ref) for :

First child = RR :0.87(CI:0.79–0.96)

Second child = RR:0.90(0.76–1.05)

Third child = RR: 0.74(0.58–0.94)

Forth child = RR : 0.65(0.39–1.08)

Race/ethnicity, subsample membership, and urban residence

Obese men compared to normal weight (Ref) for :

First child = RR :0.78(CI:0.69–0.88)

Second child = RR:0.78(0.67–0.91)

Third child = RR: 1(0.79–1.26)

Forth child = RR : 0.91(0.77–1.07)

6

Kahn 2019

BMI at 4 years ≥ 85 percentile compared to < 85 percentile:

Sperm < 15 million/ml: OR:1.7(CI:0.55-5)

Progressive motility < 32%: OR:1.4(0.67–2.8)

Sperm morphology < 4% normal: OR:1.4(0.6–3.1)

Birth weight, gestational age, abstinence time, and age at examination

BMI at 20s years ≥ 25 kg/m² compared to < 25 kg/m²:

Sperm < 15 million/ml: OR:1.4(CI:0.54–3.6)

Progressive motility < 32%: OR:2.4(1.3–4.4)

Sperm morphology < 4% normal: OR:1.9(0.94–3.8)

7

Lake 1997

Obese women at 23 year were less likely to conceive within 12 months of unprotected intercourse after adjustment for confounders (RR = 0.69)

Social class at birth, 7 and 23 y; smoking at 23 y; parental education; parity; menstrual problems at 16 y; and age of menarche

8

Laru 2021

Obesity at ages 7–10 years: Decreased fecundability at age 31 : RR = 2.05(CI:1.26–3.35), Childlessness at age 51 years : RR = 1.15(CI:0.68–1.94)

Marital status at ages 31 and 46 years, and smoking and education level at age 31 years

Obesity at ages 11–15 years: Decreased fecundability at age 31 : RR = 2.04(CI:1.21–3.44), Childlessness at age 51 years : RR = 1.97(CI:1.19–3.27)

9

Polotsky 2010

Obese group compared to normal BMI group:

Nulliparity: OR:2.84(1.59–5.10)

Adult BMI, history of nongestational amenorrhea, marital status, ethnicity, study site, and measures of socioeconomic status

Obese group compared to normal BMI group:

Nulligravidity: OR:3.93(2.12–7.26)

10

Ramlau-Hansen 2010

BMI tertile: Low BMI (12.8522–15.2348)

Medium BMI (15.2355–16.1941)

High BMI (16.2174–22.5851)

Childhood (ages 5–8 years) BMI were not significantly associated with semen quality at age 18–21.

Semen quality: Sperm concentration (millions/mL), Semen volume (mL), Sperm total count (millions), Percent normal morphology sperm, Percent motile sperm

Men with the 33% highest childhood BMI had 15% lower sex hormone binding globulin ([adjusted mean for SHBG (nmol/L): Low BMI: 30.8 (26.7, 35.4), Medium BMI: 28.8 (25.0, 33.0), High BMI: 26.2 (22.6, 30.1), p value: 0.005])

Season (summer/winter), history of diseases of the reproductive organs (cryptorchidism, hypospadias, varicocele, hydrocele, orchitis, and chlamydia combined into one variable, present or not present), smoking and maternal smoking during pregnancy. The semen outcome variables were additionally adjusted for abstinence time and spillage during collection of the sample. The results on motility were also adjusted for time from ejaculation to analysis. The blood sample outcome variables were, in addition to season, diseases of the reproductive organs, and own and maternal smoking, also adjusted for time of day of blood sampling.

11

Rich-Edwards 1993

Multi variate RR for obese group compared to controls

Ovulatory infertility = RR:2.7(CI:2-3.7)

Age, year of birth, ethnicity, frequency of physical activity at ages18-22, smoking status at ages 15–19, alcohol use at ages 18–22, DM and use of OCP before age 22