Cervical mucus: ovulation discharge through the cycle

Cervical mucus is a form of vaginal discharge. It is made by the cervix, the lower, narrow end of the uterus, and leaves through the vagina (American College of Obstetricians and Gynecologists, ACOG, patient article on trying to get pregnant, 2023). Its amount and texture follow the hormones of the cycle, and unlike basal body temperature it changes before ovulation, so it can mark the fertile days while they are still ahead. It is one of several signs of ovulation.

Rising estrogen makes cervical mucus clearer, thinner and more plentiful before ovulation

ACOG describes the pattern in its guidance on fertility awareness: just before ovulation the cervix makes noticeably more mucus, and it becomes thin and slippery; just after ovulation there is less of it, and it becomes thicker and harder to notice (ACOG, 2019). The NHS guidance on periods and fertility (2023) compares the secretions around ovulation to raw egg white: thinner and stretchy. The US Food and Drug Administration (2018) links the change to estrogen, noting that a breakdown product of estrogen builds up around ovulation as the mucus turns thin and slippery.

The American Society for Reproductive Medicine (ASRM) gives the timing in its 2022 committee opinion on optimizing natural fertility. The volume of mucus rises with estrogen over the five to six days before ovulation and peaks within the two to three days before it. ACOG's patient guidance puts the peak one to two days before ovulation (ACOG, 2023). The two accounts overlap, and both place the peak inside the fertile window, which runs from five days before ovulation to the day of ovulation (Wilcox, Weinberg and Baird, 1995).

The chance of conception is highest on days with the most fertile-type mucus

In a European study of 782 women and 7,288 cycles, the chance of conception on a given day rose steadily with the quality of the mucus seen that day. Once mucus quality was taken into account, the exact day relative to ovulation made much less difference (Bigelow and colleagues, Human Reproduction, 2004). ASRM (2022) cites a retrospective study of 1,681 cycles in which about 38 in 100 cycles led to pregnancy when intercourse fell on the day of peak mucus, against about 15 to 20 in 100 on the day before or the day after. ASRM adds that clear, slippery mucus carries the highest chance but is not required for pregnancy to occur.

Cycles with consistent mucus checks were more likely to end in conception

In a US cohort of 331 women aged 30 to 44 without known infertility, cycles in which mucus was checked on more than two thirds of the relevant days were more likely to result in conception. The association held after allowing for age, how often the couple had sex and whether urine LH tests were used (Evans-Hoeker and colleagues, Fertility and Sterility, 2013). Only 6 in 100 women in the study checked consistently. The study was observational, so it shows an association and cannot prove that checking caused the higher chance.

Mucus predicted fertile days as well as temperature or LH tests in one large study

ASRM (2022) reports that, in one large study, changes in cervical mucus across the fertile days predicted the day-by-day chance of conception as well as or better than basal body temperature or urine LH monitoring, the method behind ovulation predictor kits. In a prospective study of 2,832 cycles that ASRM also cites, mucus changes predicted peak fertility more accurately than a calendar. ACOG's patient guidance says that, of the methods for detecting ovulation yourself, monitoring cervical mucus tends to be the most reliable (ACOG, 2023), and the authors of the 2013 cohort study note that checking it costs nothing.

Checking mucus means noting what is seen and felt at the vaginal opening each day

ASRM (2022) describes cervical mucus as it is detected at the vaginal opening. ACOG (2019) describes one structured scheme, the TwoDay method: secretions are checked at least twice a day, and a day counts as likely fertile if secretions were noticed that day or the day before. Two dry days in a row make pregnancy less likely.

Several things other than ovulation change how mucus looks

ACOG (2019) lists medications, feminine hygiene products, douching, sexual intercourse, breastfeeding and a pelvic examination that uses lubricant as things that can alter the appearance of cervical mucus. A change in discharge can also have causes unrelated to the cycle, and the NHS (2023) advises seeing a GP about a change in vaginal discharge that is a concern.

Using mucus to avoid pregnancy needs training, in UK and US guidance alike

This page covers mucus as a guide to conception. Using the same signs to avoid pregnancy is a different task. The NHS (2024) says fertility awareness takes 2 to 3 cycles to learn and needs to be taught by an expert such as a fertility awareness practitioner, and ACOG (2019) suggests learning it from a qualified teacher. A calendar estimate from the ovulation calculator is not a method of contraception. The page on the most fertile days sets the mucus change alongside the other signs of ovulation.