Pelvic ultrasound: how to prepare and what it shows
Two questions come up before every scan: whether to drink water, and which day of the cycle to book. Both answers depend on which probe will be used — and that is the only thing you really need to know in advance.
Two probes, two opposite preparations
A pelvic ultrasound is done in one of two ways, and the preparation for them is exactly opposite.
| Transvaginal | Through the abdomen | |
|---|---|---|
| Bladder | empty — use the toilet before the scan | full — 0.5–1 l of water an hour before |
| Resolution | markedly better: the probe sits centimetres from the uterus | poorer, the image passes through a layer of tissue |
| When it is used | the default for sexually active women | for women who have not had intercourse, for children, for large masses |
| Duration | 10–15 minutes | 10–15 minutes |
You do not need to fast for either. It is worth cutting down on foods that cause bloating the day before, though: gas in the bowel obscures the ovaries, and that is the commonest reason a scan comes out incomplete.
Which day of the cycle
Days 5–10, counting from the first day of your period, is the default answer. The lining is thin then and does not hide small changes inside the uterus, and the ovaries are quiet, so a dominant follicle will not be mistaken for a cyst.
When a different timing is chosen:
- Assessing the lining because of abnormal bleeding — the second half of the cycle, to see it fully developed.
- Tracking ovulation — a series of scans from day 9–10.
- Suspected pregnancy, pain, bleeding — straight away, whatever the day.
- After menopause — any day; there is no cycle any more.
A scan during a period is possible and sometimes indicated, but heavy bleeding makes the uterine cavity harder to see. If it is not urgent, wait until the bleeding stops.
What the doctor looks at
| Organ | What is assessed |
|---|---|
| Uterus | position, size, wall structure, fibroids, congenital anomalies |
| Endometrium | thickness and structure — the key measure with bleeding and when planning a pregnancy |
| Ovaries | size, follicle count, cysts, corpus luteum |
| Fallopian tubes | not visible when normal; visible when dilated or fluid-filled |
| Pouch of Douglas | free fluid — a small amount can be normal |
What the scan finds
- Fibroids — their number, size and, more importantly, their position relative to the cavity: that is what determines symptoms and treatment.
- Endometrial polyps — a common cause of spotting between periods.
- Ovarian cysts — functional ones that resolve over two or three cycles, and ones that need further investigation.
- A polycystic ovarian appearance — on its own this is not a diagnosis of PCOS: symptoms and hormone tests are needed too.
- Endometriomas — the one form of endometriosis clearly visible on ultrasound.
- Hydrosalpinx — a dilated, fluid-filled tube.
- Uterine anomalies — a septum, a bicornuate uterus.
- Pregnancy, including ectopic.
What an ultrasound will not show
This is the commonest misunderstanding: imaging shows shape, not everything.
- Whether the tubes are open — that needs an HSG or HyCoSy.
- Cells — dysplasia and changes on the cervix do not show on ultrasound; that needs cervical screening and colposcopy.
- Peritoneal endometriosis — small deposits away from the ovary are usually invisible.
- Infection — that is diagnosed from a swab, not from an image.
- The nature of a mass with certainty — the scan says what looks suspicious; histology decides.
How often
Once a year if nothing is troubling you, alongside the examination and cervical screening. More often when there is something to watch: a fibroid, a cyst, hormone treatment, an IUD, trying to conceive.
You get the report immediately after the scan. Keep the previous ones: with a focal change what matters is not the size itself but whether — and how fast — it is growing.
Short answers
Do I need to drink water before a pelvic ultrasound?
Only before an abdominal scan — 0.5–1 l an hour beforehand. For a transvaginal scan the bladder should be empty.
Which day of the cycle should I book the scan?
Days 5–10 by default, counting from the first day of your period. For assessing the lining because of bleeding — the second half of the cycle. With pain, bleeding or suspected pregnancy — straight away.
What does a pelvic ultrasound show?
Fibroids, endometrial polyps, ovarian cysts, a polycystic ovarian appearance, endometriomas, hydrosalpinx, uterine anomalies and pregnancy, including ectopic.
Does a transvaginal scan hurt?
No. The probe is thinner than a standard speculum. It can be uncomfortable with inflammation or painful endometriosis — say so straight away.
This text describes the general picture and does not replace a consultation. Timings and indications differ from woman to woman and are decided at the appointment, from your history and test results.