The biology of human sex differences

Definition of Infertility and Indications for Fertility Evaluation: Consensus Document

Minjeur M et al., 2026 · Journal of Restorative Reproductive Medicine · Free to read

Infertility is a clinical condition that is recognized by the symptom of an inability to conceive through sexual intercourse or to sustain a pregnancy, with that symptom indicating underlying male and/or female pathology. This definition of infertility was developed through a structured, consensus-informed process involving broad stakeholder engagement. Initially, multiple definitions currently used by various medical professional organizations were reviewed, and a definition document was drafted and submitted to the Board of Directors of the International Institute for Restorative Reproductive Medicine (IIRRM). All IIRRM members were invited to provide feedback on the draft. Approximately 2,500 individuals and 44 organizations from 92 countries were then invited to review the proposed document, representing clinical, scientific, patient, policy, and advocacy perspectives. Submitted comments were reviewed thematically, with suggested revisions evaluated for clarity, clinical relevance, inclusiveness, and consistency with contemporary restorative reproductive medicine. Following this review, 3 substantive changes, 18 minor changes, and 15 citation corrections were incorporated into the final draft which resulted in a revised definition intended to better reflect the medical, social, and practical realities of modern infertility evaluation and care. Final approval by the IIRRM Board of Directors was unanimous.

Subfecundity, Infertility Treatment, and Child Neurodevelopment

Kahn LG et al., 2026 · JAMA Network Open · Free full text on PubMed Central

Increasing numbers of children are conceived using infertility treatment; concerns remain about potential effects on child neurodevelopment. To evaluate whether infertility treatment is associated with child neurodevelopment and whether such an association may be attributable to underlying subfecundity. DESIGN, SETTING, This cohort study was conducted among mother-child dyads in the National Institutes of Health Environmental Influences on Child Health Outcomes (ECHO) Cohort, with infants conceived between 1998 and 2022. Associations of subfecundity and infertility treatment with neurodevelopmental outcomes were assessed among children ages 2 to 10 years. Data were analyzed from May 14, 2025, to March 31, 2026. Subfecundity was defined as prior consultation for, treatment of, or diagnosis of infertility for either partner; at least 2 prior miscarriages; or ever having had unprotected heterosexual intercourse for 12 months without conceiving. Infertility treatment was categorized as in vitro fertilization (IVF) or non-IVF treatment. Harmonized caregiver responses to the Strengths and Difficulties Questionnaire and the Child Behavior Checklist yielded continuous raw scores for externalizing and internalizing problems. The total raw Social Responsiveness Scale (SRS) score quantified autism-like symptoms. Caregivers reported physician diagnosis of autism spectrum disorder (ASD) and attention deficit/hyperactivity disorder (ADHD). Among 15 382 mother-infant dyads, there were 14 191 unique maternal participants (mean [SD] age at delivery, 30.9 [5.33] years; 8780 parous participants [57.1%]). ASD and ADHD were diagnosed in 876 offspring (7.6%) and 819 offspring (7.1%), respectively. In generalized linear models, subfecundity was associated with higher externalizing problem and SRS scores among all pregnancies (externalizing problems: b = 0.47 [95% CI, 0.14-0.81]; SRS score: b = 1.08 [95% CI, 0.01-2.14]) and when restricted to natural conceptions (externalizing problems: b = 0.45 [95% CI, 0.07-0.83]; SRS score: b = 1.12 [95% CI, -0.09 to 2.34]). Offspring of parents with subfecundity had higher odds of ASD (overall: odds ratio [OR], 1.27 [95% CI, 1.03-1.57]; natural conceptions: OR, 1.31 [95% CI, 1.04-1.64]). Children conceived via non-IVF treatment had higher odds of ADHD compared with those conceived via natural conception with subfecundity (OR, 1.77 [95% CI, 1.16-2.68]) or without subfecundity (OR, 1.54 [95% CI, 1.05-2.25]). There were no significant associations for IVF treatment. In this large US cohort study, subfecundity was associated with elevated scores for caregiver-reported symptoms of behavioral problems and higher odds of ASD diagnosis, independent of infertility treatment. Non-IVF treatment was associated with ADHD, warranting further research into specific indications for treatment that may increase risk of offspring neurodevelopmental problems.

Potential increase of the U.S. total fertility rate resulting from restorative treatment of unresolved subfertility: a simulation study

Stanford JB et al., 2026 · Frontiers in Reproductive Health · Free full text on PubMed Central

Background The total fertility rate (TFR) in most developed countries has been declining for decades. In the United States (U.S.), the total fertility rate has remained below replacement level since 2007. Subfertility affects at least 15% of women or couples over their reproductive lifespan and contributes to reduced TFR. Restorative reproductive medicine (RRM) is a medically based approach to subfertility care that can be delivered in primary care settings to increase live birth rates. Objective To estimate the theoretical impact of use of RRM among subfertile couples in the United States. Methods We conducted a simulation study. Model inputs included the number of women of reproductive age in the United States by 5-year age groups; current age-specific and total fertility rates; the proportion of women in each age group with subfertility; estimated spontaneous live birth rates among women with subfertility; and age-specific crude live birth rates with RRM treatment. We evaluated fifteen scenarios including sensitivity analyses: two different varying assumptions for spontaneous conception (25% vs. 50%), two levels of RRM utilization among subfertile women (20% vs. 50%), three different estimates of the number of subfertile women who would be potentially eligible for RRM treatment, and 4 different levels of effectiveness (live birth) from RRM treatment. Results The baseline TFR in the United States was 1.77 during 2015-2019, and 13.5% of women ages 20-44 were estimated to have subfertility. In a conservative scenario (50% spontaneous births; 20% RRM utilization; married women trying to conceive for at least 12 months, 20.7% RRM live births), the TFR increased to 1.79, representing a 1.0% relative increase (absolute +0.02). In an optimistic scenario (25% spontaneous births; 50% RRM utilization; all subfertile women), the TFR increased to 2.02, a 14.5% relative increase (absolute +0.26), approaching replacement-level fertility. Conclusion Simulation results suggest that expanding access to RRM within primary care settings could meaningfully increase the U.S. TFR, by reducing unresolved subfertility. Realizing this potential would require policy and health system changes to address workforce capacity, insurance coverage, and equitable access. These findings underscore the potential contribution of non-IVF fertility care pathways in addressing population-level fertility decline.

Restorative Reproductive Medicine Restricting Access to Care

Minjeur M et al., 2026 · JAMA

Overview of the biological aspects of the fertile period

France JT, 1981 · Int J Fertil

The fertile period of the human menstrual cycle consists of those days on which sexual intercourse can result in a pregnancy. Its duration is determined by the functional life span of the gametes within the female reproductive tract. Various mechanisms control gamete transport and survival in the reproductive tract of the human female. The ovarian hormones estradiol and progesterone have an important role in regulating these mechanisms. The nature of cervical mucus and its governing influences on sperm transport and survival following coitus are of prime importance in defining the fertile days of the menstrual cycle. Man's early concepts of the fertile period were often based on erroneous theories of the female reproductive cycle. It is only since the late 1920's that a true understanding of ovulation and the menstrual cycle has evolved. Current approaches in natural family planning to recognizing the fertile and infertile days of the menstrual cycle are discussed and evaluated.

Cervical mucus patterns and the fertile window in women without known subfertility: a pooled analysis of three cohorts

Najmabadi S et al., 2021 · Hum Reprod · Free full text on PubMed Central

What is the normal range of cervical mucus patterns and number of days with high or moderate day-specific probability of pregnancy (if intercourse occurs on a specific day) based on cervical mucus secretion, in women without known subfertility, and how are these patterns related to parity and age? The mean days of peak type (estrogenic) mucus per cycle was 6.4, the mean number of potentially fertile days was 12.1; parous versus nulliparous, and younger nulliparous (<30 years) versus older nulliparous women had more days of peak type mucus, and more potentially fertile days in each cycle. The rise in estrogen prior to ovulation supports the secretion of increasing quantity and estrogenic quality of cervical mucus, and the subsequent rise in progesterone after ovulation causes an abrupt decrease in mucus secretion. Cervical mucus secretion on each day correlates highly with the probability of pregnancy if intercourse occurs on that day, and overall cervical mucus quality for the cycle correlates with cycle fecundability. No prior studies have described parity and age jointly in relation to cervical mucus patterns. STUDY DESIGN, SIZE, This study is a secondary data analysis, combining data from three cohorts of women: 'Creighton Model MultiCenter Fecundability Study' (CMFS: retrospective cohort, 1990-1996), 'Time to Pregnancy in Normal Fertility' (TTP: randomized trial, 2003-2006), and 'Creighton Model Effectiveness, Intentions, and Behaviors Assessment' (CEIBA: prospective cohort, 2009-2013). We evaluated cervical mucus patterns and estimated fertile window in 2488 ovulatory cycles of 528 women, followed for up to 1 year. PARTICIPANTS/MATERIALS, SETTING, Participants were US or Canadian women age 18-40 years, not pregnant, and without any known subfertility. Women were trained to use a standardized protocol (the Creighton Model) for daily vulvar observation, description, and recording of cervical mucus. The mucus peak day (the last day of estrogenic quality mucus) was used as the estimated day of ovulation. We conducted dichotomous stratified analyses for cervical mucus patterns by age, parity, race, recent oral contraceptive use (within 60 days), partial breast feeding, alcohol, and smoking. Focusing on the clinical characteristics most correlated to cervical mucus patterns, linear mixed models were used to assess continuous cervical mucus parameters and generalized linear models using Poisson regression with robust variance were used to assess dichotomous outcomes, stratifying by women's parity and age, while adjusting for recent oral contraceptive use and breast feeding. MAIN The majority of women were <30 years of age (75.4%) (median 27; IQR 24-29), non-Hispanic white (88.1%), with high socioeconomic indicators, and nulliparous (70.8%). The mean (SD) days of estrogenic (peak type) mucus per cycle (a conservative indicator of the fertile window) was 6.4 (4.2) days (median 6; IQR 4-8). The mean (SD) number of any potentially fertile days (a broader clinical indicator of the fertile window) was 12.1 (5.4) days (median 11; IQR 9-14). Taking into account recent oral contraceptive use and breastfeeding, nulliparous women age ≥30 years compared to nulliparous women age <30 years had fewer mean days of peak type mucus per cycle (5.3 versus 6.4 days, P = 0.02), and fewer potentially fertile days (11.8 versus 13.9 days, P < 0.01). Compared to nulliparous women age <30 years, the likelihood of cycles with peak type mucus ≤2 days, potentially fertile days ≤9, and cervical mucus cycle score (for estrogenic quality of mucus) ≤5.0 were significantly higher among nulliparous women age ≥30 years, 1.90 (95% confidence interval (CI) 1.18, 3.06); 1.46 (95% CI 1.12, 1.91); and 1.45 (95% CI 1.03, 2.05), respectively. Between parous women, there was little difference in mucus parameters by age. Thresholds set a priori for within-woman variability of cervical mucus parameters by cycle were examined as follows: most minus fewest days of peak type mucus >3 days (exceeded by 72% of women), most minus fewest days of non-peak type mucus >4 days (exceeded by 54% of women), greatest minus least cervical mucus cycle score >4.0 (exceeded by 73% of women), and most minus fewest potentially fertile days >8 days (found in 50% of women). Race did not have any association with cervical mucus parameters. Recent oral contraceptive use was associated with reduced cervical mucus cycle score and partial breast feeding was associated with a higher number of days of mucus (both peak type and non-peak type), consistent with prior research. Among the women for whom data were available (CEIBA and TTP), alcohol and tobacco use had minimal impact on cervical mucus parameters. LIMITATIONS, We did not have data on some factors that may impact ovulation, hormone levels, and mucus secretion, such as physical activity and body mass index. We cannot exclude the possibility that some women had unknown subfertility or undiagnosed gynecologic disorders. Only 27 women were age 35 or older. Our study participants were geographically dispersed but relatively homogeneous with regard to race, ethnicity, income, and educational level, which may limit the generalizability of the findings. Patterns of cervical mucus secretion observed by women are an indicator of fecundity and the fertile window that are consistent with the known associations of age and parity with fecundity. The number of potentially fertile days (12 days) is likely greater than commonly assumed, while the number of days of highly estrogenic mucus (and higher probability of pregnancy) correlates with prior identifications of the fertile window (6 days). There may be substantial variability in fecundability between cycles for the same woman. Future work can use cervical mucus secretion as an indicator of fecundity and should investigate the distribution of similar cycle parameters in women with various reproductive or gynecologic pathologies. STUDY FUNDING/COMPETING INTEREST(S): Funding for the three cohorts analyzed was provided by the Robert Wood Johnson Foundation (CMFS), the Eunice Kennedy Shriver National Institute of Child Health and Human Development (TTP), and the Office of Family Planning, Office of Population Affairs, Health and Human Services (CEIBA). The authors declare that they have no conflict of interest. N/A.

Revisiting the fertile window

Stanford JB, 2015 · Fertil Steril

Ovulation occurs on 1 day during each menstrual cycle (even if multiple follicles are involved), and the several days preceding ovulation are when intercourse is most likely to result in pregnancy. Collectively, the potentially fertile days up to and including the day of ovulation are called the “fertile window” (1). Since the 1930s, several biomarkers of the fertile window have been investigated for the purposes of empowering women and couples seeking to use fertility-awareness based methods or natural family planning to avoid pregnancy, as well as to expand understanding of human reproductive physiology.

Human cervical mucus: relationship between biochemical characteristics and ability to allow migration of spermatozoa

Morales P et al., 1993 · Hum Reprod

Cervical mucus is produced throughout the menstrual cycle. Sperm migration, however, is possible only during the periovulatory period of the cycle. Cervical mucus is also produced during the amenorrhoeic post-partum period. Post-partum mucus is very similar to luteal phase mucus except that it can allow sperm migration. In this study, mucus samples obtained from all these periods were classified according to their capacity to allow sperm migration. The biochemical characteristics of mucus samples that did (peri-ovulatory and 40% of post-partum samples) and did not (luteal and 60% of post-partum samples) allow sperm migration were then compared. Mucus samples with positive sperm migration showed the highest percentage of water and lowest protein and glycoprotein concentration (per ml of mucus). In addition, post-partum mucus samples with positive sperm migration showed lower concentrations of proteins and glycoproteins than post-partum mucus samples that did not allow sperm migration. However, the amount of glycoproteins per mg of protein was similar between post-partum samples that were positive and negative for sperm migration. These data suggest that the carbohydrate composition of the glycoproteins is playing a key role in the ability of cervical mucus to accept spermatozoa.