Physiological Factors Affecting Fertility: From Sterility to Breastfeeding

Fertility may sound like a simple biological story – two people, a healthy body, and a baby. But the reality is far more layered. Population studies treat fertility as the outcome of a chain of physiological events, each of which can speed things up, slow them down, or stop them altogether. From the early years of adolescence to the long months of breastfeeding after a child is born, the body itself sets quiet rules about when conception is possible. Understanding these physiological factors helps explain why birth rates differ across regions, communities, and even households living on the same street.

Why physiological factors matter in fertility studies 🔗

In demography, scholars often separate the social reasons for having fewer children (marriage age, contraception, education) from the biological ones. The biological side is what we call physiological factors – the body-based conditions that decide whether conception is even possible. Demographers have shown that biological constraints alone can lower a woman’s maximum feasible fertility from around 35 births to roughly 15 over her lifetime, a concept known as total fecundity. Within this biological ceiling, four broad forces matter most: sterility, the temporary inability to conceive after childbirth, the suppressing effect of breastfeeding, and pregnancy losses through abortion or stillbirth.

These factors are deeply connected to the framework of proximate determinants of fertility, popularized by John Bongaarts. He grouped the biological and behavioural pathways into a small set – including onset of permanent sterility, postpartum infecundability, spontaneous intrauterine mortality, and induced abortion – through which all social and economic conditions must finally pass to influence fertility outcomes.

Sterility and its different forms 🔗

Sterility, in demographic language, is the inability of a couple to produce a live birth. It is not always permanent and not always present from the start of marriage. Population scientists usually classify it into three types: adolescent, primary, and secondary sterility.

Adolescent sterility 🔗

When a girl is married very young, her reproductive system may not yet be mature enough to support conception or carry a pregnancy. This biological gap between menarche and full reproductive capacity is called adolescent sterility. In societies where child marriage was historically common, this phase explained why the first birth often came several years after marriage – not because of contraception, but because the body itself was not ready. Studies note that the interval between cohabitation and the first child tends to be longer in such cases, since the girl is not yet physically developed to bear a child. As age at marriage rises across the country, adolescent sterility plays a smaller role than it once did, but it still has lingering effects in pockets where early marriage continues.

Primary sterility 🔗

Primary sterility refers to couples who have never had a child despite trying. According to the WHO’s working definition, it means the inability to conceive within two years of exposure to pregnancy among women aged 15 to 49 who are sexually active, not using contraception, and not lactating. A community-based study in central India placed primary infertility at around 8.9 per cent among married women of reproductive age. Causes can include anatomical issues, hormonal disorders such as polycystic ovary syndrome, untreated infections, or male-factor issues like low sperm count. A study from Mysore even highlighted that sexually transmitted infections, especially HSV-2, are strongly linked to primary infertility in young women.

Secondary sterility 🔗

Secondary sterility describes couples who have had at least one child but are unable to conceive again. This may follow a difficult delivery, an infection like pelvic inflammatory disease, a botched abortion, surgery, or simply advancing age. Recent analyses using the National Family Health Survey reveal an unsettling pattern: primary infertility has declined while secondary infertility has risen from 19.5 per cent in 1992-93 to 28.6 per cent in 2015-16. This shift matters for population planners because it tells them that fertility loss today is increasingly happening after the first child, not before it.

The period of infecundability after childbirth 🔗

Having a baby does not immediately reset the body to a fertile state. There is a natural pause, known as the period of postpartum infecundability, during which conception is biologically unlikely. This pause is one of the most important physiological brakes on fertility, especially in settings where modern contraception is not widely used.

Postpartum amenorrhea 🔗

The most visible sign of this pause is postpartum amenorrhea – the absence of menstrual periods after delivery. Without menstruation, ovulation is usually absent too, which means conception cannot take place. The duration of postpartum amenorrhea varies sharply between women. A community survey of Manipuri women found the median duration of postpartum amenorrhea was 5.7 months, influenced strongly by place of residence, infant survival, and breastfeeding duration. Demographers point out that on average, postpartum infecundability adds roughly one and a half months to the interval between births, on top of the nine months of pregnancy itself.

How breastfeeding extends the pause 🔗

Breastfeeding is the single biggest factor that lengthens postpartum amenorrhea. The act of suckling triggers the release of prolactin, a hormone that suppresses ovulation. In the absence of breastfeeding, the interruption in fertility lasts less than two months, but with lengthy and frequent breastfeeding it can stretch to one or two years. This is why traditional societies that practised long, intensive breastfeeding tended to have naturally longer birth intervals, even before contraception was available.

Research grounded in Indian data found that breastfeeding reduced the risk of conception by around 47 per cent, and after statistical adjustments the effect rose to 63 per cent. A separate hazards analysis from Assam and Uttar Pradesh showed that even after menstruation returned, every additional month of continued breastfeeding lowered the chance of conception. International evidence supports this: the Bellagio Consensus concluded that full or nearly full breastfeeding during lactational amenorrhea offers about 98 per cent protection against pregnancy in the first six months after childbirth. This protective effect is the foundation of the Lactational Amenorrhea Method (LAM), recognised as a short-term natural family planning approach.

However, the method has limits. Once a baby starts taking supplementary feeds, or once menstruation returns, the contraceptive effect weakens quickly. Clinicians caution that lactation alone is unreliable for preventing conception after the ninth week postpartum when feeding patterns are irregular, which is why couples wanting to delay the next pregnancy are usually advised to add another contraceptive method.

Pregnancy loss as a physiological barrier 🔗

Not every conception results in a live birth. Pregnancy loss – whether through abortion or stillbirth – is another physiological factor that shapes overall fertility. Bongaarts included spontaneous intrauterine mortality and induced abortion as two of his seven proximate determinants for precisely this reason.

Spontaneous and induced abortion 🔗

Spontaneous abortion, popularly called miscarriage, ends a pregnancy without intervention, usually in the first trimester. Causes include chromosomal abnormalities, hormonal imbalance, infections, uterine fibroids, or chronic illness in the mother. Induced abortion is a deliberate medical termination of pregnancy and is legal under specific conditions through the Medical Termination of Pregnancy Act. At the population level, induced abortion does not lower fecundability (the ability to conceive) but it does reduce fertility by ending an ongoing pregnancy, as demographers have long noted. On average, intra-uterine mortality adds about two months to the interval between two births. Repeated abortions, especially if performed unsafely, can also lead to secondary sterility by damaging the uterus or causing infection.

Stillbirth 🔗

A stillbirth is the death of a foetus after a defined period of gestation (commonly 28 weeks in Indian reporting) but before delivery. It is one of the most tragic outcomes of pregnancy and a serious public health concern. According to recent global burden estimates, the country contributed the highest number of stillbirths globally in 2021, at around 397,300, even after a 53 per cent decline in the stillbirth rate between 2000 and 2019. From a demographic angle, stillbirths reduce live births directly and can also delay or discourage future pregnancies. Studies based on the Annual Health Survey of nine populous states found significant variation in stillbirth risk linked to maternal age, anaemia, poor antenatal care, and rural residence.

Putting it all together: the birth interval 🔗

One way demographers integrate all these physiological forces is by studying the birth interval – the time between two consecutive live births. A natural birth interval includes: nine months of pregnancy, a few months of postpartum amenorrhea (longer with breastfeeding), a waiting time to the next conception, and possibly extra months added by any pregnancy loss before a successful birth. When sterility shortens the reproductive span at either end (adolescent at the start, primary or secondary in between), or when stillbirths and abortions interrupt the middle, the realistic number of children a woman can have falls well below the theoretical biological maximum.

This is why countries with high prevalence of untreated infections, anaemia, malnutrition, or unsafe abortion practices often show higher levels of secondary sterility and longer effective birth intervals – even when their crude birth rates appear high. Improving maternal nutrition, treating reproductive tract infections, encouraging exclusive breastfeeding, and providing safe abortion services are therefore not just health interventions; they are demographic interventions that shape national fertility patterns.

Why this matters for population policy 🔗

Modern family welfare programmes treat physiological factors as both a constraint and an opportunity. Programmes that promote exclusive breastfeeding for the first six months simultaneously protect infant health and extend postpartum infecundability, which helps couples space their births. Reproductive health services that treat infections early, manage PCOS and thyroid disorders, and offer assisted reproductive technology when needed help reduce primary and secondary sterility. Strengthening antenatal care and skilled birth attendance is the single most effective way to bring stillbirth numbers down further.

In short, the physiology of fertility is not destiny. It interacts continuously with nutrition, health systems, culture, and personal choices. The body sets the stage, but society writes much of the script.

What do you think? Should family planning programmes in our country place stronger emphasis on treating secondary infertility, given how much it has risen in recent decades? And how should public health messaging balance the proven benefits of long-term breastfeeding with the reality that lactational amenorrhea alone is not always a reliable contraceptive?

References

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