People often think of fertility in absolute terms — as in, someone is either fertile or infertile. But the reality is fertility exists on a spectrum, with a substantial gray zone of what’s often called subfertility between the two endpoints. The distinctions are more than semantics. Ultimately, “it comes down to time to pregnancy,” says Joseph A. Hill, MD, a reproductive endocrinologist and infertility specialist and president of the Fertility Centers of New England in Boston.

Fertility, Subfertility, Infertility: What’s in a Name?

Infertility is defined as trying to become pregnant for a year, with properly timed sexual intercourse (during the most fertile phase of a woman’s menstrual cycle), without success. This can be due to male factors (such as sperm abnormalities, obstructions, or ejaculatory dysfunction) or female factors (such as blocked fallopian tubes, not ovulating regularly, hormonal irregularities, or endometriosis) or it can be due to a combination or unexplained factors. With infertility, “you have less than a 5 percent chance of being able to conceive on your own,” Dr. Hill says.

Subfertility Is an Extended Period of Unwanted Nonconception

By contrast, the term subfertility describes a prolonged time span of trying to become pregnant (or an extended period of unwanted nonconception) that hasn’t reached a year, notes Mary Ellen Pavone, MD, an associate professor of obstetrics and gynecology in the division of reproductive endocrinology and infertility at the Northwestern University Feinberg School of Medicine in Chicago.

With “normal” fecundity or fertility, a woman’s “ability to become pregnant is about 15 to 20 percent per month,” with properly timed intercourse, Dr. Pavone says. Even so, with intercourse during the most fertile phase of a woman’s menstrual cycle, 80 percent of pregnancies will occur within six cycles among women under age 30, according to research published in the journal Human Reproduction. After six unsuccessful cycles, about 20 percent of couples are considered at least slightly subfertile, but one-half of these couples will go on to conceive naturally or spontaneously in the next six cycles — before the definition of infertility sets in.

Risk Factors and Reasons a Couple Might Be Subfertile

Research published in March 2015 in the Archives of Gynecology and Obstetrics found that the primary factors that impair the chances of pregnancy in subfertile women are older age, endometriosis, diabetes, ovarian dysfunction, polycystic ovary syndrome (PCOS), and previous infection of the genitourinary tract. By contrast, previous use of hormonal contraceptives, a previous pregnancy or birth, and a history of progesterone therapy at any time were associated with increased pregnancy rates among women diagnosed with subfertility.

Other Health Issues That Can Interfere With Fertility

In addition, some underlying health conditions, such as certain thyroid or adrenal gland disorders, can contribute to subfertility, notes Brooke Hodes-Wertz, MD, a reproductive endocrinologist and fertility specialist at the NYU Langone Fertility Center in New York City. And women with autoimmune diseases, such as rheumatoid arthritis (RA) or lupus, are more likely to have premature ovarian insufficiency (egg loss) that can compromise their chances of getting pregnant.

What’s more, a study published in August 2017 in the journal Arthritis Care & Research found that women with rheumatoid arthritis are more likely to be diagnosed with subfertility, perhaps because they use more nonsteroidal anti-inflammatory drugs (NSAIDs) before conception.

Meanwhile, lifestyle factors also can play a role in subfertility. The biggest contributor to otherwise unexplained subfertility is smoking (both active and passive smoking). In women, smoking reduces ovarian reserve (the number of eggs in the ovary at any given time) and compromises the functioning of the fallopian tubes and the uterine environment, according to a review published in February 2016 in the journal The Obstetrician & Gynaecologist.

Extra Weight Can Get in the Way of Getting Pregnant

Another contributor to subfertility, which is related to genetics as well as lifestyle, is obesity. When it comes to body weight, obesity can take a toll on a woman’s fertility by reducing the quality of her eggs and impairing implantation, according to a study published in August 2017 in the journal Human Reproduction. Plus, being significantly overweight or obese can lead to hormonal irregularities that can interfere with ovulation, Pavone says.

Weight Loss May Help Some Women Get Pregnant

In some women who are obese or overweight and have subfertility, slimming down can improve the odds of spontaneous conception. In fact, in a study published in August 2017 in the journal Human Reproduction, researchers investigated whether an intense weight reduction program (that led to an average loss of 21 pounds over 12 weeks) in obese women with infertility would affect their outcomes with subsequent in vitro fertilization (IVF). As it happened, 4 times more live births were achieved through spontaneous pregnancies among the women who lost weight and did IVF, compared with those who did IVF alone.

Improving Your Odds of Getting Pregnant

After spending years trying not to get pregnant, once you decide you want to have a baby, you might expect it to happen right away with unprotected sex. But that’s not realistic. Still, that doesn’t mean you have to try for years to make a baby before seeking help.

If you’ve been monitoring ovulation by tracking your cervical mucus or using an over-the-counter ovulation predictor and having unprotected intercourse without getting pregnant, talk to your doctor about it after six months if you’re over 35, or after a year if you’re under 35, Hill advises.

Addressing Fertility Problems and the Causes of Subfertility

Depending on your underlying health status or other symptoms, your doctor may recommend looking for anatomic factors (such as polyps, fibroids, or endometriosis) that could be interfering with pregnancy, checking your hormone levels, or sending your guy for a semen analysis. Once the likely culprit is revealed, a course of action to address (and hopefully reverse) the problem can be created.

In some cases, what’s called “expectant management” may be recommended if it seems possible that a couple could conceive spontaneously after being diagnosed with subfertility. Women with mild endometriosis or uterine growths might be candidates for laparascopic surgery to make the uterus a more hospitable environment for implantation. In others, ovulation induction or IVF might be recommended, depending on how long a couple has been trying to conceive and the woman’s age.

Mind Your Biological Clock: Age Matters

The important thing is to be proactive if you’re been experiencing subfertility. “Fertility is precious and finite,” Hill says. “It can’t just be claimed anytime someone gets around to it.”