My great grandmother had 15 and my grandmother ( her daughter) had one . The jockeying for attention, pseudo-parenting by the older kids , and the bullying by the older kids sorta explains that . Not one of the 15 had more than 3. They didn’t like growing up being part of large family. My great grandmother would be sick with each of those 15 pregnancies and she was stuck with that because she couldn’t refuse her husband. “Good” women didn’t practice birth control back then ( late 19th century) She probably would have if it hadn’t been illegal. I suspect that my grandmother did as she was a teenager/ young adult in the roaring 20s
The factors around fertility are actually quite complex, and we have good data for the last century or so. Social stress is associated with early menarche (starting your period), and growing up in a single parent home is too, independantly thereof. Excess nutrition in childhood, but pre-natal starvation, are also associated with early menarche. Urbanisation also shifted fertility patterns.
The early 20th century was a roller coaster ride, of good times in 1900, 1920, 1950s; and shortages and disease in 1910s, and 1930 till the 50s, etc. Further, many households lost husbands in the wars, and the social disruption of the following years didn't help. On average, the menarche shifted earlier in the 20th century, but marriages later; children born out of wedlock remained low till late in the century, but sexually transmitted infections dramatically increased; and the entirety has to do with declining fertility rates. The failure rates of early family planning was very high, as vulcanised rubber condoms were crude, spermicides ineffective, early IUDs ineffective or dangerous, and hormonal contraceptive only available from the 60s. Of course, Abortion also increased during that time, but the estimated mortality of an abortion was about 30-40%, so this is insufficient to explain the decline in fertility or family size.
So the fertility period often shifted earlier, but women still only married later and most still only procreated in wedlock, so a shorter period of optimum fertility; with increasing factors like urbanisation and irregular nutrition trends, decline of fathers in the household, increased STIs, etc. Family sizes dramatically trended downward from the 1880s onwards, but that can only be partly attributed to birth control which was not effective or safe till the 60s, and has more to do with shifting social and biological fertility patterns in the population.