Fecundity refers to the biological capacity of a woman, a couple, or a population to reproduce. It denotes the potential for childbearing under normal physiological conditions, irrespective of whether a birth actually occurs. In demographic analysis, fecundity is distinguished from fertility, which refers to the actual number of live births produced. While fertility is an observed outcome measured through indicators such as crude birth rate or total fertility rate, fecundity is a biological potential influenced by factors such as age, health, nutrition, and genetic characteristics. Fecundity varies across the reproductive lifespan of women, generally beginning at menarche and ending at menopause, with the highest probability of conception occurring during the twenties and early thirties. Not all women are equally fecund at all times; temporary conditions such as illness, breastfeeding, or hormonal imbalances may reduce fecundability, which is the probability of conception during a menstrual cycle. Thus, fecundity provides the biological foundation upon which social, cultural, and behavioral factors operate to shape fertility levels in a population. In the field of population studies, the relationship between fecundity and fertility is central to understanding population growth and change. Although biological capacity sets the upper limit of reproduction, actual fertility is regulated by a variety of intermediate variables. One of the most influential frameworks explaining how social and biological factors affect fertility was developed by the demographer John Bongaarts. Bongaarts’ model of the proximate determinants of fertility identifies the direct mechanisms through which socio-economic and cultural factors influence reproductive outcomes. Instead of examining broad structural variables such as education or income directly, the model focuses on measurable biological and behavioral factors that immediately affect exposure to the risk of conception and childbearing. According to Bongaarts, there are four principal proximate determinants of fertility: marriage or sexual exposure, contraception, induced abortion, and postpartum infecundability. These determinants operate directly on fertility by influencing the frequency of sexual intercourse, the likelihood of conception, and the probability that a pregnancy results in a live birth. Each determinant is represented by an index that quantifies its fertility-inhibiting effect relative to a situation of maximum natural fertility. The first determinant is marriage or sexual exposure. In many societies, particularly traditional ones, childbearing occurs primarily within marriage. Therefore, the proportion of women in reproductive ages who are married or in a stable sexual union significantly affects fertility levels. If women marry at younger ages and remain married throughout their reproductive years, exposure to the risk of pregnancy increases, leading to higher fertility. Conversely, delayed marriage, non-marriage, divorce, or separation reduces exposure and thus lowers fertility. Bongaarts developed an index of marriage (Cm) to measure the extent to which non-marriage reduces fertility from its biological maximum. In societies undergoing modernization, rising female education and employment opportunities often lead to delayed