The cervix is at the bottom of the uterus and is connected to the uterus at the isthmus. Stimulated by the ovarian hormones the cervix produces several types of cervical mucus. The diagram at the right illustrates the various crypts from which the mucus secretions emanate. These have a crucial role in fertility and in the general health of the woman. Cervical mucus discharges at the vulva and can be felt and observed by the woman. This is the basis of the Billings Ovulation Method.

A commentary on each stage in the ovarian cycle is given below with an associated sequence of  images.

An accompanying page provides an animation of the sequence.

Colour codes used in the cervix images

G mucus

L mucus

P mucus

S mucus

Menstrual flow

Ovarian Phase

Cervix Behaviour

Isthmus & Cervix 

Menstruation

The G mucus plug dislodges to allow menstrual flow.

Menstruation

Any presence of mucus is masked by the menstrual flow when ovulation is early.

Menstruation

Menstrual flow lightens and  thick G mucus accumulates in the cervical canal. Basic Infertile Pattern may be identified even in the presence of spotting.

Basic Infertile Pattern

After menstruation the narrow cervix is occluded by G mucus.
Sperm are kept out in the vagina where their survival time is very short.

Basic Infertile Pattern

The narrow cervix is occluded by G mucus. Sperm are kept out in the vagina where their survival time is very short.

First point of Change
Potential Fertility

G mucus diminishes aided by the liquifying effect of P2 mucus.
Sperm may now enter the cervix where sperm life is prolonged.

Potential Fertility

G mucus diminishes, L mucus increases, S mucus begins, providing nourishment and channels for the transport of sperm.

Potential Fertility

S and L mucus increase.

Potential Fertility

G mucus further diminishes, L mucus increases, S mucus increases.  The changes in the fertile pattern are observed at the vulva and are caused by the changing proportions of the mucus types.

Strings due to the combination of L and S mucus may appear.

Potential Fertility

The liquifying effect of the P mucus on the S and L mucus strings is pronounced.  Strings of mucus may disappear leaving only a slippery sensation at the vulva

Peak of Fertility

Ovulation is imminent. The ovum may survive up to 24 hours after ovulation.

The cervix has reached it peak development for conception. Conditions are most favourable for the selection and rapid transport of high quality sperm. The vulva feels slippery and swollen.

Peak+1
Potential Fertility

In most cases ovulation occurs either on the peak day or on this day. The ovum may survive up to 24 hours. Channels for sperm transport still exist.

The G mucus begins to form in the lower crypts. The vulva is no longer wet or slippery.

Peak+2
Potential Fertility

G mucus increases, S and L mucus diminish.

Ovulation may occur as late as this day. The ovum may survive up to 24 hours. Channels for sperm transport exist in diminshing numbers.

The vulva is no longer wet or slippery.

Peak+3
Potential Fertility

G mucus increases. S and L mucus still present in small amounts.

Channels for sperm transport still exist.

Conception is possible if the ovum is present.

The vulva is no longer wet or slippery.

Luteal Phase 

Cervix occluded by G mucus. The egg is dead. Conception is no longer possible.

The woman is infertile.

Luteal Phase (Infertile)

The cervix channel is narrow and it is occluded by G mucus.

Menstruation

The G mucus plug is removed to allow menstrual flow.