Cervix function and anatomy: where the cervix is and what it does

In one line: the cervix is the firm, narrow lower end of the uterus, sitting where the vagina ends, commonly somewhere around 8 to 15 cm (roughly 3 to 6 inches) in from the entrance, and its job is to make mucus, let menstrual blood out, let sperm through most readily around ovulation and make the canal far less passable for the rest of the month, hold a pregnancy in, and then soften and open for birth.

The cervix is one of the few parts of the body that most women first hear named during a screening appointment or in the last weeks of a pregnancy, and are then expected to understand straight away. It is a small structure, no bigger than the end of a small bottle, sitting at the upper end of the vagina. Yet it changes more across a single month, and far more across a pregnancy, than almost any other tissue in the pelvis. Understanding what it is and what it does makes everything else about cervical health easier to follow, from mucus patterns to the wording on a screening letter.

This article sets out where the cervix is located and what it is made of, then works through what it actually does: producing and changing mucus, letting menstrual blood out, allowing sperm through during the fertile days and blocking it for the rest of the month, staying shut and forming a mucus plug in pregnancy, and then softening, thinning and opening in labour. It also covers how it changes in pregnancy, after a birth and after the menopause, what an examination and screening look for, and the common cervical findings in plain terms. This is patient information, not advice about your own case, which belongs with your own doctor.

Where is the cervix located, and what is it?

The cervix is the lower part of the uterus: the narrow end that opens into the vagina. If the uterus is pictured as a small upside down pear, the cervix is its stalk. It sits at the upper end of the vaginal canal, commonly somewhere around 8 to 15 cm (roughly 3 to 6 inches) in from the vaginal opening, pointing downwards and backwards towards the base of the spine. In an adult it is in the region of 2.5 to 3.5 cm long, with sizes varying between women and with age, and it feels firm to touch, closer to the tip of the nose than to the soft walls around it.

How far in it sits, and which way it points, depends partly on how the uterus is lying. A uterus tipped forwards carries the cervix a little further back in the vagina, while a uterus tipped backwards brings it further forward, and both are ordinary variants rather than problems. The height of the cervix in the vagina also shifts through the cycle and rises during pregnancy, so a cervix that feels high one week and easier to reach the next is behaving normally.

Seen through a speculum, the part on view at the end of the vagina is the ectocervix, with a small dimple or slit in the middle of it. That opening is the external os. Behind it runs the cervical canal, a passage only a few millimetres wide, which rises to a second opening called the internal os, where the canal meets the cavity of the uterus. The structure is mostly dense connective tissue with a smaller amount of muscle, and that composition is exactly what allows it to stay closed under the weight of a pregnancy and still open when it has to.

The cervical canal, and the tissue that lines it

The canal is not a plain tube. It is lined by tall glandular cells, the same family of cells that line the cavity of the uterus, folded into small crypts and clefts. Those crypts are where cervical mucus is produced, and they also act as a short term reservoir: sperm can be held there for a time rather than passing straight through. This glandular lining is called the endocervix, and it looks redder and more velvety than the surface outside, because the tissue is thinner and the blood vessels sit closer to the surface.

The outer surface, the ectocervix, is covered by flat, layered cells of the same kind that line the vagina. They are tougher and paler, built to tolerate friction and an acidic environment. The border where the two kinds of lining meet is called the squamocolumnar junction, and the strip of tissue around it, where one cell type is gradually replaced by the other, is the transformation zone.

That border is not fixed. It sits further out on the surface in adolescence, in pregnancy and while taking hormonal contraception, and it retreats up into the canal after the menopause. Because the transformation zone is where cells are actively turning over, it is also where cell changes tend to begin, which is why it is the specific area a screening sample is aimed at. The practical consequences of that are covered further down this page.

When a cervical symptom needs assessment the same day

Almost everything described on this page can wait for an ordinary appointment. A small number of situations cannot, and they belong near the beginning rather than buried at the end, because the interval between a symptom starting and being assessed is what changes how it turns out. If any of the following applies, act on it now rather than reading further, and use the timing given in each line.

• Heavy vaginal bleeding, soaking through a pad an hour for two hours in a row, or passing clots larger than a golf ball, or any bleeding alongside feeling faint, dizzy or clammy, or collapsing: go to an emergency department or call your local emergency number now.
• Any bleeding during pregnancy, at any stage and however light: contact your maternity unit, early pregnancy service or doctor the same day, not at the next routine appointment. A blood streaked mucus show in late pregnancy is described further down this page, but if you cannot tell a show from bleeding, ring them anyway.
• Pain low down on one side with an out of character bleed in anyone who could be pregnant: same day assessment. If a pregnancy test is positive, or has not been done, this needs to be seen urgently because of the possibility of an ectopic pregnancy. A negative test makes ectopic pregnancy unlikely but does not explain the pain, so it still needs assessing the same day.
• A gush or a steady trickle of fluid from the vagina in pregnancy, or regular tightenings before 37 weeks: contact your maternity unit straight away, not later the same day, because this may mean the membranes have opened or the cervix is changing early, and both are assessed urgently.
• Fever, or feeling shivery and unwell, with lower abdominal or pelvic pain, with or without an offensive discharge, at any time and with or without a recent procedure, and particularly in the days after a birth, a miscarriage or a procedure on the cervix or uterus: same day assessment. Any one of these features is enough, because this can mean infection has spread above the cervix.

None of these can be settled by reading about them. Separately from this list, bleeding after sex, bleeding between periods and any bleeding after the menopause should all be assessed rather than watched, even though they are rarely emergencies. Those three patterns have their own section at the end of this article, because they are the commonest reason a cervix is examined and the easiest ones to put off.

What does the cervix do?

The cervix has one structural job, to be a controllable doorway between the uterus and the outside world, and it performs that job differently depending on what the hormones of the moment are asking of it. Across a lifetime it does five recognisable things, and most of the confusion about the cervix comes from mixing them up.

• Produces and changes mucus. The glands of the canal make cervical mucus continuously, and alter its amount and structure through the cycle.
• Lets menstrual blood out. The canal is the only route for the lining of the uterus to leave the body, which is why a tight or scarred cervix can make periods more painful.
• Lets sperm through at the right time, and blocks it the rest of the month. This selectivity is done almost entirely by the mucus rather than by the opening itself.
• Keeps a pregnancy where it belongs. It stays firm and closed, and the mucus thickens into a plug that acts as a physical and chemical barrier.
• Opens for birth. Towards the end of pregnancy and in labour it softens, shortens and then dilates enough for a baby to pass through, before closing again afterwards.

None of this is under conscious control. The cervix is not a muscle you can tighten or release at will, and the sensation it carries is limited and poorly localised, which is why pressure on the cervix is often felt as a deep, vague ache rather than a sharp pain in one spot. What drives the changes is the hormonal state: oestrogen opens things up and thins the mucus, progesterone closes them down and thickens it. Everything that follows in this article is a variation on that single theme.

Cervical mucus: the part that changes most across the cycle

Cervical mucus is the most changeable thing the cervix produces, and the change is not random. As oestrogen rises in the first half of the cycle, the glands of the canal produce more mucus and its protein mesh loosens. In the days around ovulation it becomes clear, slippery and stretchy, often compared to raw egg white, and that open structure lets sperm swim through and survive in the crypts of the canal for a few days rather than minutes. This is the physical basis of the fertile window.

After ovulation, progesterone from the corpus luteum does the opposite. The mucus becomes scant, thick and tacky, the mesh tightens, and the canal returns to being a barrier. The same effect explains why progestogen based contraception and pregnancy both produce thick mucus. How to read these changes in practice, and how reliable they are as a signal, is covered in our article on cervical mucus and fertility.

Cervical mucus is also the main reason healthy women notice discharge at all. Mixed with vaginal secretions and shed cells it becomes the white or clear discharge that varies through the month. What is not ordinary is discharge that is green or grey, smells offensive, comes with itching, soreness or pain, or is persistently blood stained. Those features point towards infection or inflammation rather than a normal mucus change, and they should be examined and tested rather than guessed at, because the treatment depends entirely on what is found.

How the cervix changes across the menstrual cycle

Position, firmness, the size of the opening and the mucus all move together through the cycle, driven by oestrogen and then progesterone. The table below sets out the usual direction of travel. It describes tendencies rather than a timetable: cycles vary in length between women and from month to month, and plenty of women never notice any of it.

Self examination of the cervix is not a test, and nothing in this table diagnoses anything. It can be useful background for someone already tracking a cycle, and unhelpful for someone looking for certainty about a particular day. If the reason for watching is difficulty conceiving, the more informative conversation is with your own doctor, who can look at cycle length, ovulation and the rest of the picture together rather than at the cervix alone.

The cervix in pregnancy: closed, plugged and sometimes measured

Once a pregnancy is established, progesterone keeps the cervix firm and the canal shut, and the mucus thickens into the mucus plug: a barrier that sits in the canal and carries antibacterial proteins as well as being physically dense. At the same time the cervix becomes softer at its tip and considerably richer in blood vessels. That is why light bleeding after sex or after a screening sample is more common in pregnancy, and also why it still needs reporting the same day rather than explaining away at home.

In some pregnancies the length of the cervix is measured in millimetres on a transvaginal scan, usually in the middle months and usually because of a previous very early birth, a previous procedure on the cervix, or a finding on an earlier scan. Where the cervix is found to be shorter than expected for that stage, the options an obstetric team may raise include a stitch placed in the cervix, progesterone treatment, or closer monitoring without either, and which of those is discussed depends on the history and the rest of the findings rather than on the measurement alone. Whether cervical length is measured in every pregnancy or only where there is a specific reason to look also differs between maternity services. Which approach applies to you, what a measurement means in your case and what should follow from it are decisions for the obstetrician looking after the pregnancy, who has the rest of the history in front of them.

Towards the end of pregnancy the plug is lost, sometimes as a single jelly like piece and sometimes as thickened, blood streaked discharge over several days. This is known as a show, and it means the cervix is changing rather than that labour has started: it usually comes away in the days just before labour or at the very start of it, and occasionally a little earlier. A show is not the same thing as bleeding, but the two are not always easy to tell apart at home. If you see fresh red blood, more than a streak of blood in mucus, blood soaking a pad, or you simply are not sure which you are looking at, treat it as bleeding in pregnancy and contact your maternity unit the same day, as in the list above. Any leaking of fluid belongs in that list too.

Softening, effacing and dilating: the cervix in labour

Before the cervix can open, it has to change character. The dense collagen that held it closed for months is remodelled, water content rises, and the tissue becomes soft and pliable instead of firm. This is called ripening, and it happens over the final weeks rather than in a single moment. It is the reason a cervix can be examined late in pregnancy and described as favourable or unfavourable, which is a description of texture, position and length rather than a prediction of when labour will start.

Effacement is the next step: the cervix shortens and thins, drawing up into the lower part of the uterus until what was a thick, tubular structure becomes a thin rim. Clinicians often describe it as a proportion of the way from thick to paper thin. Dilatation is the opening itself, recorded in centimetres across the os, and conventionally described as reaching about 10 cm when it is fully open. The two processes overlap, and the order differs: in a first pregnancy the cervix tends to efface substantially before it dilates much, while in later pregnancies the two often happen together.

How long any of this takes varies enormously, and the definitions of normal progress differ between professional bodies, which is one reason women are given different accounts of what counts as slow. Recognising labour is covered in detail in the patient information from bodies such as ACOG, listed in the sources below, and the judgement about your own labour sits with your maternity team. After the birth the cervix closes again over the following days and firms up over some weeks, although the shape of the opening usually stays slightly different afterwards.

After childbirth, and after the menopause

The cervix of a woman who has given birth looks different from one that has not. The external os, a small round dimple beforehand, usually becomes a transverse slit, and the tissue sits a little more open and flush at the end of the vagina. None of this needs correcting and none of it affects later screening. In the weeks after a birth the cervix is still soft and the canal still open enough to let lochia drain, which is why heavy fresh bleeding, an offensive discharge or a fever in that period is assessed the same day rather than watched.

After the menopause the direction reverses. With less oestrogen the tissue thins and becomes less elastic, the cervix can flatten against the end of the vagina, and the canal can narrow, occasionally enough that a screening sample or a procedure becomes technically harder. Dryness means the surface bleeds more readily on contact. These changes sit alongside the wider picture described in our article on the stages and symptoms of menopause, and local treatment is available for the symptoms if they are troublesome, which is a conversation for your own doctor.

Two practical points follow. Cervical screening does not stop at the menopause: invitations continue to the upper age limit used by whichever programme you are in, and having had no new partners does not change that. And bleeding after the menopause is never written off as thinning tissue without being looked at, however light it is and however long ago the last period was. That is the subject of the final section.

What a speculum examination shows, and why the transformation zone matters

A speculum examination exists because the cervix is one of the few internal organs that can simply be looked at. The speculum holds the vaginal walls apart so the ectocervix and the external os are visible, and the examiner notes the appearance of the surface, any discharge, and whether the tissue bleeds on light contact. A screening sample, where one is due, is taken with a small brush rotated against the surface. Swabs may be taken if infection is a question. A bimanual examination, with a hand on the abdomen, assesses the size and mobility of the uterus and any tenderness.

Comfort is a legitimate part of the examination rather than an afterthought. Telling the examiner while it is happening that something hurts is useful information rather than a nuisance, because there are adjustments that can be made once it is said. If a previous examination was difficult, distressing or impossible, that is worth raising before the next one starts, since it changes how it is approached. The practical adjustments themselves are set out in the questions at the end of this page.

The transformation zone is the target of the sample for the reason given earlier: it is where glandular lining is being replaced by surface lining, where the cell turnover happens, and therefore where changes arise. If the zone has retreated into the canal, as it does with age, a sample can miss it and may need repeating. Where screening suggests a change, the next step is usually colposcopy, which is the same kind of examination carried out with a magnifying light and, where needed, a small biopsy.

What is tested, from what age and how often all differ between countries, and those differences are real rather than a sign that one programme is wrong. Many programmes now test the sample first for high risk human papillomavirus and examine the cells only if the virus is found. The NHS invites women aged 25 to 64 and, when no high risk virus is detected, repeats the test every 5 years, while ACOG patient information describes starting at 21, with cytology every 3 years from 21 to 29 and primary HPV testing or co-testing every 5 years from 30 to 65. Vaccination against the virus is offered separately and does not replace screening. Follow the interval your own doctor gives you.

Common cervical findings, in plain terms

A number of findings are reported often enough that they deserve a plain explanation, because the names sound more alarming than the conditions usually are. What follows is what each one means, not what should be done about yours, which depends on your age, your symptoms and what else was found at the time.

• Cervical ectropion, also written as cervical erosion, means the glandular lining of the canal is visible on the outer surface. Nothing is eroded and nothing is damaged. It is common in younger women, on hormonal contraception and in pregnancy, and it can cause clear discharge or bleeding on contact, which is why it is treated only when it is a nuisance.
• Cervical polyps are small, usually benign growths arising from the lining of the canal. They can bleed, particularly after sex, and are often removed and sent for examination to confirm what they are. Their counterpart inside the uterus is covered in our article on uterine polyps and fertility.
• Nabothian cysts are blocked mucus glands: smooth, firm bumps on the surface of the cervix, often noticed incidentally. They are harmless and ordinarily need nothing done.
• Cervicitis means the cervix is inflamed, looking red and often with discharge. It can follow an infection, including a sexually transmitted one, and the point of the examination is to test for what is causing it. Treatment is prescribed by a doctor once that is known; over the counter or home remedies are not a substitute, because the wrong assumption delays the right treatment. Left untreated, an infection causing cervicitis can spread up to the uterus and tubes as pelvic inflammatory disease, which can affect fertility, so it is tested and treated rather than waited out.
• Cell changes found on screening mean altered cells in the surface layer of the cervix, not a growth or a tumour. The screening report grades them by how abnormal the cells look, usually described as borderline, low grade or high grade. If a biopsy is taken at colposcopy, the grade used there (CIN 1, 2 or 3) is different: it describes how far through the thickness of the surface layer the changes reach. Either way, whether the changes are monitored or treated is decided in the colposcopy clinic from the grade, the HPV result and your age. Many of the mildest changes settle on their own.

A phrase on a report is a description, not a diagnosis. The same words carry different weight at 24 and at 64, and in someone with no symptoms compared with someone bleeding after sex. If you are holding a result you do not understand, ask the doctor who requested it to go through it with you, including what follow up it does and does not require. That conversation is a reasonable use of an appointment, not a trivial one.

Bleeding after sex, between periods or after the menopause: get it looked at

Three bleeding patterns point towards the cervix often enough that each one is a reason to be examined rather than to wait and see. The first is bleeding after sex. The commonest explanations are an ectropion, a polyp or inflammation from an infection, all of which are manageable once identified, but the only way to tell which it is, or that it is none of them, is to look. This is covered in more depth in our article on bleeding after sex.

The second is bleeding between periods. A little spotting around ovulation in an otherwise regular cycle has a well recognised hormonal explanation. A new, repeated pattern of bleeding at other times, or bleeding that is unpredictable, is different and should be assessed. The third is any bleeding after the menopause, which includes a single episode, a pink or brown stain, and bleeding in someone taking hormone therapy who has been told to expect none. NHS information is explicit that postmenopausal bleeding should be checked rather than waited out.

Examination is advised in all three situations for one reason: the great majority of these bleeds have a benign explanation, but cancer of the cervix and of the lining of the uterus can present with exactly this kind of bleeding, and both are more straightforward to deal with when they are found early. A normal screening result does not answer new bleeding either, because screening looks for a virus and for cell changes in women without symptoms, while symptoms are assessed on their own merits. Contact your own doctor without waiting, and say clearly which pattern of bleeding it is, since that determines how soon you are seen. For bleeding after the menopause in particular, NHS guidance is that your GP should refer you on and that you should not have to wait more than two weeks to be seen by a specialist, so say if you are offered something later than that.

Related Reading

• Cervical Mucus and Fertility: How It Changes Across Your Cycle
• Bleeding after sex with no pain: causes, assessment and when to be seen
• Ovulation Explained: When It Happens and How to Find Your Fertile Window
• What is menopause? The stages, symptoms and age range explained
• Uterine polyps and fertility: endometrial polyps, symptoms and removal

Sources and references

This article is supported by the following independent, authoritative sources:

• Cleveland Clinic, Cervix: anatomy, function and conditions
• MedlinePlus, Cervix
• NHS, Cervical screening
• ACOG, Cervical cancer screening
• ACOG, How to tell when labor begins
• NHS, Postmenopausal bleeding