Thanks for clarifying! I will estimate how much of the decline in fertility rates in the U.S. and other developed nations can plausibly be explained by greater access to and use of birth control, especially the pill. I’ll focus on unplanned pregnancy rates before the pill (1950s-early 1960s), during the transition period (1970s-1980s), and today. I’ll also estimate how much higher today's total fertility rates (TFR) would likely be if unplanned pregnancy rates had remained at pre-pill levels. I’ll prioritize high-quality historical studies and demographic reconstructions.I’ll get started and will let you know when my findings are ready!

Impact of Birth Control on Declining Fertility Rates

Pre-Pill Era (1950s–Early 1960s): High Unplanned Pregnancy Rates

In the decades before modern contraceptives became available, a large share of pregnancies in the U.S. and other developed countries were unplanned. Couples had limited means to control family size (relying on abstinence, withdrawal, or ineffective methods), leading to many more pregnancies than intended. Surveys from the early 1960s indicate that over half of U.S. births were unintended at the time of conception – about 55% of births in 1965 were not plannedwww.ncbi.nlm.nih.gov. Notably, 24% of births were explicitly “unwanted” (i.e. the parents did not desire any more children) in that periodwww.ncbi.nlm.nih.gov. This implies many families in the pre-pill era ended up with more children than they had initially hoped or planned for.Premarital pregnancies were often likewise unplanned and stigmatized in the 1950s. Out-of-wedlock birth rates were low (only ~5% of U.S. births in 1960 were to unmarried womenwww.washingtonexaminer.com) not because premarital conception was rare, but because “shotgun marriages” were common. In the late 1960s, roughly 60% of premarital pregnancies led to a rapid marriage before the birthwww.washingtonexaminer.com. In other words, when an unmarried woman became pregnant, social norms virtually required the couple to marry. This practice masked the true extent of unplanned pregnancies by folding many of them into marriage. Nevertheless, the prevalence of shotgun marriages underscores how frequently unplanned pregnancies occurred in the pre-pill era and how strong the pressure was to legitimize those pregnancies. Families often “made the best” of an unplanned conception by having an earlier or larger family than they otherwise would have.Overall, in the 1950s and early 1960s, unplanned childbearing was a major contributor to the high birth rates of the time. The U.S. Total Fertility Rate (TFR) during the baby boom peaked around 3.5–3.7 children per woman in the late 1950s, and even by 1960 was roughly 3.3–3.6. A significant portion of this was driven by mistimed or unwanted pregnancies. Many married couples had an extra one or two “surprise” children because reliable birth control was unavailable to them. Likewise, premarital conceptions often simply resulted in earlier marriages and childbearing. In summary, before the pill, both premarital and within-marriage unplanned pregnancies were rampant, inflating fertility rates beyond what couples might have actually desired.

Transitional Period (1970s–1980s): Widespread Contraception Amid Cultural Lag

The 1960s and 1970s brought revolutionary changes in contraceptive access. The first oral contraceptive pill was approved in 1960, and over the next decade it became widely used by married women in the U.S. (helped by the Supreme Court’s Griswold v. Connecticut decision in 1965 which legalized birth control for married couples). By the early 1970s, the pill and newer methods like intrauterine devices were increasingly available to unmarried women as well (after Eisenstadt v. Baird in 1972). As a result, by the 1970s most sexually active couples in developed countries had access to effective contraception, dramatically reducing unintended births. Demographic data show a steep drop in unplanned childbearing during this transition: the share of U.S. births that were unintended fell from 55% in the mid-1960s to about 30% by 1982www.ncbi.nlm.nih.gov. In particular, births that were truly unwanted (not just mistimed) became far less common, dropping from 24% of all births in 1965 to only 7% by 1982www.ncbi.nlm.nih.gov. This indicates that many couples who would have “accidentally” had a fourth or fifth child in the pre-pill era were successfully stopping at two or three children by the 1970s thanks to contraception.Even as birth control use became mainstream, cultural and social stigma persisted in certain groups during the 1970s and 1980s, leading to higher unplanned pregnancy rates for those populations. For example, the Catholic Church’s official opposition to artificial contraception (articulated in 1968’s Humanae Vitae) meant some devout Catholic couples avoided the pill. In countries or regions with strong religious influence, this slowed the uptake of modern birth control. (Ireland is a case in point – contraception remained illegal there until 1979, and Irish fertility stayed high (~3+ children per woman) well into the 1970s before dropping once birth control was legalized in the 1980s.) In the U.S., unmarried young women initially had less access to contraception or were reluctant to use it. Through the 1970s, there was still significant stigma around premarital sex and teen girls obtaining birth control. Consequently, unintended pregnancy remained relatively common among teens and single women even as it plummeted among married adults. Data from the late 1980s illustrate this divide: by 1990, a whopping 73% of births to never-married American women were the result of unintended pregnancieswww.ncbi.nlm.nih.gov, and an even higher 86% of births to unmarried teenagers were unplannedwww.ncbi.nlm.nih.gov. These figures show that certain subpopulations (young, unmarried, low-income) were slower to benefit from contraceptive advances, due to either limited access, less consistent use, or lingering stigma.Another factor in this era was the introduction of legal abortion (Roe v. Wade in 1973 in the U.S.), which also reduced the number of unplanned births. Some unintended pregnancies that would have resulted in unplanned births in the past were terminated instead. This contributes to the declining proportion of births that were unplanned. (It’s worth noting, however, that our focus is on birth control; abortion played a role in reducing births, but the primary driver of fewer unintended births was improved contraception preventing those pregnancies in the first place.)By the mid-1980s, the overall fertility rate in the U.S. had dropped to around 1.8–1.9 children per woman, less than half the baby boom level. Much of this decline corresponded with the prevention of unintended pregnancies. Indeed, among married women, the number of “excess” births they hadn’t intended to have was dramatically lower than a generation prior. Most couples in this period were having roughly the number of children they wanted, and no more. Demographers noted that the decline in unplanned births accounted for a large portion of the overall fertility decline from the 1960s to the 1980swww.ncbi.nlm.nih.govwww.ncbi.nlm.nih.gov. For instance, one analysis found that the intended childbearing rate among U.S. married women stayed roughly constant at around 1.3 children per woman between the 1960s and early 1980s, while the unintended childbearing rate fell by about one whole birth per woman over that period – explaining most of the drop in total fertility. In short, the pill and other contraceptives enabled families to avoid the previously common “extra” births, especially higher-order births that used to occur by accident.It’s important to note that not all developed countries experienced this transition at the same time or pace, due to cultural differences. Generally, Western Europe followed a similar pattern: the advent of the pill in the 1960s led to rapid fertility decline in the 1970s. For example, the United Kingdom’s TFR fell from about 2.9 in 1964 (just as the pill became available) to around 1.7 by the late 1970s, a drop largely attributed to widespread contraceptive use. Nordic countries, Canada, Australia, and Japan (which utilized other contraception and legalized abortion early) all saw fertility declines coinciding with the spread of modern birth control. However, in more traditional societies or those with restrictions on contraception (such as Spain, Italy, and Ireland in the 1960s–70s), high fertility persisted a bit longer, only dropping once social norms and laws changed. By the 1980s, virtually all developed nations had seen significant reductions in unplanned pregnancies and total birth rates as contraceptive use became the norm.

Contemporary Era: Planned Births and Lower Unintended Pregnancy

Today, in the 2000s and 2010s, effective birth control is widely available and broadly accepted across developed countries. This has brought unplanned pregnancy rates down to the lowest levels on record – though they are not zero. In the United States, about 45% of all pregnancies are still unintended (mistimed or unwanted)www.brookings.edu, but because many of those pregnancies do not result in a birth (some end in abortion or miscarriage), the share of births that are unplanned is lower. Roughly one-third of U.S. births today are from unplanned pregnancieswww.brookings.edu. In other words, about 65–70% of children are now planned by their parents, compared to only ~45% in the early 1960s. This represents a huge change in family planning dynamics. The unintended birth rate in the U.S. has continued to inch downward in recent years, aided by better contraceptive technologies (like long-acting reversible contraceptives such as IUDs and implants), more comprehensive sex education, and policies like no-cost contraception under health insurance. In fact, researchers note that U.S. unintended pregnancy rates reached an all-time low in the 2010swww.brookings.edu. The decline has been especially notable among teens and unmarried women, the groups that historically had the highest unplanned pregnancy rateswww.brookings.edu. Even so, young and low-income women still experience disproportionately higher unintended pregnancy and birth rates than older, married, or more affluent womenwww.brookings.eduwww.ncbi.nlm.nih.gov, indicating ongoing gaps in access and education.Other developed countries tend to have even lower rates of unplanned pregnancy and birth than the U.S., thanks to stronger social support systems and contraceptive use. For instance, many Northern and Western European nations report that a solid majority of births are planned. (For example, countries like Germany or the Netherlands have very low teen pregnancy rates and high contraception usage, resulting in fewer accidental births.) Nonetheless, even in these countries, a non-trivial minority of births are unintended – often on the order of 10–20% unwanted or mistimed. Modern couples nearly everywhere have fewer children than past generations, and crucially, most children today are born because their parents chose to have them, not because contraception failed or was unavailable.At present, the Total Fertility Rates in most developed nations range from about 1.3 to 1.9 children per woman, well below the 1950s–60s levels. The United States is at the upper end of that range (recent U.S. TFR ~1.6–1.7 as of 2020nchstats.com), whereas the European Union average is around 1.5. These low fertility rates reflect a combination of factors – later marriage and childbearing, higher education and workforce participation for women, economic considerations, and the near-universal ability to avoid having more children than desired. It’s now common for couples to stop at one or two children because that is their preference, and they have the tools (contraception) to make that choice reliably.To put contemporary figures in perspective: The U.S. general fertility rate (births per 1,000 women of childbearing age) in 2023 was about 54.5, which is less than half the rate in 1957 (122.9 per 1,000 at the peak of the baby boom)usafacts.org. Demographers attribute this long-term decline to a mix of social changes, but improved birth control is a central element. As one analysis summarizes, from 1950 to 2024 the U.S. TFR fell by roughly 47% (from ~3.1 to 1.6), driven in large part by increased access to contraception (along with education, women’s workforce participation, etc.)nchstats.com. The story is similar across advanced economies – fertility declined sharply as societies transitioned to smaller, planned families.

How Much of the Fertility Decline Is Attributable to Birth Control?

It is challenging to assign an exact percentage of the fertility decline to a single factor, but we can make a reasonable estimate of how much modern birth control (especially the pill) has reduced birth rates by preventing unplanned pregnancies. One approach is to compare the number of births that would occur at high unplanned pregnancy rates to the number of births that actually occur with widespread contraception.In the early 1960s (pre-pill), American women on average had about 3.5 children, of which roughly 1.9 were not intentionally planned (as 55% of births were unintended) and ~1.6 were planned. By contrast, today American women have about 1.7 children on average, of which roughly 0.5–0.6 might be unplanned (around 33% of 1.7) and ~1.1–1.2 are planned. This means that effective contraception has eliminated on the order of one unintended birth per woman (on average) compared to the pre-pill era. If today’s Americans were still experiencing unplanned pregnancies at 1960s rates, the total fertility rate would likely be significantly higher. We can estimate this by holding the number of planned births constant while raising the unplanned fraction to old levels. For example, assuming women today still only intend to have ~1.1–1.2 children on average (which roughly matches surveys of desired family size), but 55% of births were accidental, the TFR in the U.S. would be around 2.5 (since the 1.2 planned births would constitute 45% of all births in that scenario) instead of the actual ~1.7. In other words, modern birth control is plausibly preventing on the order of 0.8 additional births per woman in the U.S., which accounts for a very large share of the gap between mid-20th-century and current fertility rates. By this rough calculation, roughly 40–50% of the drop in TFR from ~3.5 to ~1.7 can be directly explained by the reduction in unplanned births due to contraception. The remainder of the decline is explained by changes in desired family size and timing (people now choose to have fewer children, later in life, for various reasons).We see similar impacts in other developed nations. Most Western European countries had TFRs around 2.5–3.0 in the late 1950s; those have fallen to ~1.5 today. Much of that decline occurred in the 1970s once the pill became available, indicating that fewer “extra” births were occurring. While cultural and economic factors also lowered desired family size, the ability to reliably avoid unintended pregnancies was crucial in translating those desires into reality. If unplanned pregnancy rates had not fallen, many European countries would likely still have fertility near or above replacement level (around 2.5 instead of 1.5, hypothetically). In short, the pill and other contraceptives gave women and couples control to have only the number of children they want – no more surprise fifth child or hurried marriage due to an unexpected pregnancy – and this has been a cornerstone of the fertility decline in developed societies.The table below summarizes estimated unplanned pregnancy metrics and fertility rates across eras in the United States, illustrating birth control’s impact: