Cervical Rules for BBT

Cervical Rules for BBT

 Follow these rules to check your cervical mucus for indications of ovulation.

Mucus Rules

  1. Ideally the mucus should be checked through the day and recorded each evening.
  2. Check for the mucus at the vaginal opening before urinating, either by:

(a) wiping with paper before urinating

(b) Inserting a finger and feeling at the opening of cervix, [This method is best as you can catch mucus changes early and also note cervical changes.]

  • Record the cervical secretions according to the three broad categories of mucus (or the absence of it). It is important to remember that mucus patterns are individual and some women may not recognise all of the following types of mucus.

Types of cervical mucus:

Type 1: a feeling of dryness and no visible cervical mucus.

  • This is found in the infertile phase at the beginning of the cycle after a period, and again after ovulation until the next period starts.

Type 2: scanty, thick, white, sticky, creamy or yellowish mucus which gives a sensation of moistness or stickiness; it then becomes thinner, milky, cloudy or slightly stretchy and a moist sensation may be present.

  • This is the transitional stage under the influence of increasing oestrogen from the growing follicles, and marks the beginning (and end) of the fertile phase
first 3 images for mucus

Type 2 cervical mucus: Sticky, thick, white, creamy, milky, slightly stretchy (transition from the infertile to fertile phase)


Type 3: is profuse, stretchy (like raw egg white) or very thin, transparent and slippery, blood-tinged or brownish mucus. The physical sensation is wet, slippery or moist.

  • This is the most fertile type of mucus, which is seen for a few days before ovulation and in some cases continues for 1-2 days after ovulation
  • Even if no mucus is seen but there is a distinct wet sensation, this counts as the most fertile type of mucus
  • Under the influence of oestrogen, cervical mucus can contain 96-98% of water and be so watery it is not visible
2nd 3 images for mucus

Type 3 cervical mucus: Stretchy, white/transparent (beginning of highly fertile phase)

3rd of the 3 mucus images

Type 3 cervical mucus: Very stretchy (“spinnbar”), transparent, like raw egg-white, watery, abundant (highly fertile phase)

Cervical mucus function

The different types of cervical fertile mucus have a variety of functions, they:

  • Protect the sperm from the hostile vaginal pH and flora
  • Supply the sperm with energy
  • Encourage the transfer of sperm from the vagina to the uterus
  • Filters abnormally shaped sperm

There is a gradual change in cervical mucus from an infertile (type1) to the fertile mucus (types 2&3) in the follicular phase in response to rising levels of oestrogen that are produced by the growing follicles. The mucus quickly changes back to the infertile mucus quality, following the rapid drop in oestrogen, and rise in progesterone levels after ovulation.

Peak Mucus Day

The last day of your highest-quality mucus is called the “peak mucus day” and this day is closely associated with the time of ovulation. Peak mucus day should be recorded as a marker for the ‘fertile window’.

There are different glands inside the cervix which produce the 4 different ‘types’ of cervical mucus. Should you wish to it is possible to dry the mucus on a smooth surface, (such as a glass microscope slide), in order to be certain what type of mucus you are producing.

Below are pictures of the different types of cervical mucus and an explanation of their function. Their names are ‘G’ (gestational – ie during pregnancy), L (loaf), S (string) and P (peak). [i]



Fig 1. ‘L’ mucus dried sample. This has a characteristic ‘ferning’ shape to it. The function of this type is to catch and filter out some of the abnormal or poorly shaped sperm before they reach the uterus. This type of mucus feels sticky or wet, and is found in the transitional and fertile phases.

Fig 2. ‘S’ mucus dried sample. This has a characteristic ‘pine-needle’ structure to it, where the strands are lined up close and parallel. The function is to create or facilitate pathways for the sperm to follow into the uterus. This type of mucus feels stretchy and slippery like egg white and is found in the fertile phase.

Fig 3. ‘G’ mucus dried sample. This is thick, pasty and impenetrable mucus that is found in the infertile phases of the cycle and during pregnancy.

Fig 4. ‘P’ mucus dried sample. This has a characteristic ‘hexagon’ structure to it. The ‘P’ mucus is produced from glands at the top of the cervical canal, and its function is to activate the sperm as they pass through the cervix into the uterus. It feels wet and lubricating, and is found in the fertile phase.

Fig 5. This is an Electron Microscope slide of ‘L’ mucus.

Fig 6. This is an Electron Microscope slide of ‘S’ mucus.


Important things to know:

  • Fertile-type cervical mucus does NOT necessarily indicate ovulation as the egg may not actually be released.
  • Semen in the vagina or a pathological discharge may be mistaken for cervical mucus but can be distinguished, with some practice, by smell or appearance.
  • On occasion there may be a brown-red mucus from the cervix under high levels of oestrogen; spotting after intercourse at mid-cycle may be a sign of high fertility, and is most usually found in younger women. (Please note that spotting after intercourse may also be a sign of unwanted changes in the cells of the cervix, but this will also happen in the less fertile phases of your cycle).
  • Ovulation will have occurred on one of the following days: 2 days before and 2 days after the last day with the “peak mucus day” (highest-quality mucus).

Cervical Changes for BBT

Recording changes in the cervix is an alternative to cervical mucus observation. As it is generally more difficult to do it’s recommended for:

  • women who have difficulty recognising their mucus
  • women do not have clear mucus patterns
  • women who have had cervical surgery, which will reduce their secretions

Either mucus or cervical changes should be recorded; please note that recording both does not increase the chance of conception.

How to find your cervix:

The Best position is usually squatting or sitting on the toilet. It often helps to use the abdominal muscles to push the uterus down. Don’t panic if you struggle to find it at first as it takes some time to become familiar with your cervix!

It’s easiest to feel in the luteal Phase – bear down, and keep trying; the feeling varies between:

Like your nose & hard: luteal & non- fertile follicular phase

Like your chin and softer: the fertile follicular phase

Typical cervical pattern:

The infertile phase, at the beginning of the cycle and after ovulation, corresponds to Type 1 mucus:

  • the cervical os is closed
  • the cervix is hard
  • the cervix protrudes into the vagina and is easily felt with a finger

The transitional stage, is the start of the fertile phase, is when changes in the cervix are first felt, and corresponds to Type 2 mucus:

  • the cervical os opens slightly
  • the cervix becomes softer
  • appears shorter and higher in the vagina

The most fertile days corresponds to Type 3 mucus:

  • the os is open
  • the cervix is soft
  • the cervix is high in the vagina
  • it may be difficult to reach and feel wet or slippery

As with the changes in cervical mucus, the pre-ovulatory change is gradual while the post-ovulatory transition takes only 1-2 days

How to record changes in the cervical os:  ● =  closed,     ʘ = transitional,      O =  open

Cervix open and closed


Cervix in fertile phase: os is open, soft, with fertile mucus.          Cervix in infertile phase: os closed, hard, with no mucus

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