Background Teenage pregnancy is a major health problem significantly associated with negative impacts on the health of both teenage mothers and their newborn. However, little is known about teenage pregnancy from teenager's perspective, particularly in developing countries including Iran. Aim This study aimed to explore the experience of pregnancy in Iranian teenage women. Methods An interpretive phenomenological study was used as a suitable research design to conduct this research. Data were collected through individual, semi structured and in-depth interview with 11 married teenage women aged between 15 and 19 years old, primigravida with singleton pregnancy. Data were analysed through thematic analysis approach. Findings “Fast development” was the main theme that emerged from the participants’ experiences. It refers to the unexpected development process that occurs simultaneously with other important development events. Fast development consists of three themes, ‘unexpected development’, ‘development within development’, and ‘struggle with development’. Conclusion Teenage pregnant women simultaneously encounter multiple developmental challenges related to adolescence period, marriage, pregnancy, and mothering responsibilities. According to the results, fast development concept should be considered by healthcare providers in order to offer comprehensive and age-appropriate health services to pregnant teenage women for successful transition from the multiple developmental stages. Moreover, this concept will help health care providers, especially midwives, to understand how to deal with pregnant teenagers.