Fertility and reproductive behaviour in the nineteenth-century Lutheran family from the city of Poznan was characterized. Use was made of the St. Peter parish (Peterkirche) registers from 1840s to 1870s, upon which reconstructions of the individual histories of 463 families were based. Fertility was assessed on the basis of the length of between-birth intervals according to their order and age of women. Next, the age-specific fertility rates of women f(x) and Coale Index of marital fertility I-g were calculated. From a seasonal rhythm of both marital and illegitimate births, and reconstructed on the basis of these, the annual rhythm of marital and prenuptial conceptions were characterized. In the Lutheran Parish of St. Peter from the city of Poznan illegitimate births and prenuptial conceptions accounted for almost 10% and over 5% of all births, respectively. Lack of seasonality for marital births was noted here. Illegitimate births, by contrast, were characterized by a statistically significant annual rhythm: their maximum was noted in April, which was a result of conceptions during summer. The average woman with completed reproduction cycle gave birth to the first and to the last child at the ages of 29 and 40 years, respectively. She therefore used 32% of her whole reproductive period. Late age of birth of the first child was associated with late age of marriage of women (on average 28 years). An average woman with completed reproduction gave birth to 4.8 children. The average length of the protogenesic interval was 18 months. The highest fertility occurred in women aged between 20 to 29 years. The value of the I-g index was 0.67, and proved a lack of birth control rather than conscious birth regulation. Generally, Lutheran women from Poznan were characterized by fairly high reproductive potential.