This page starts with very simple information and then goes into a more detailed explanation of ectopic pregnancy. If there are words you aren't sure of which are underlined then just click on them and an explanation will be shown

• What is an ectopic pregnancy?
• How do fallopian tubes work?
• How do ectopic pregnancies happen?

What is an Ectopic Pregnancy?

Put very simply, an ectopic pregnancy means "an out-of-place pregnancy".

It happens when a woman's ovum (egg), when it has been fertilised by a man's sperm, remains "stuck" in the tube instead of moving down her fallopian tube into the womb to develop there.

How Fallopian Tubes Work

The fallopian tubes (or oviducts) are small, hollow muscular tubes, each about ten centimetres long. The outer half lies next to its ovary.

The delicate mucous membrane lining inside the tube is thrown up into folds which almost fill each tube (see diagram below). The diameter and the number of folds increase as the tube nears the ovary and forms the fimbriae - tiny finger-like projections.

In the epithelial lining of the tubes, half the cells are mucus-secreting and half have cilia - tiny hair like projections which beat gently to propel these secretions towards the uterus - rather like the cilia in the main airways of the lungs, (the bronchi) which waft a "moving carpet" of mucus up into the throat to keep dust and bacteria out of the lungs. (The lumen of the tubes is tiny by comparison and contains no air, of course.)

The muscular wall of each tube becomes thicker towards the uterus, and has a natural peristaltic action which assists the movement of mucus.

After sex with a man, sperms swim up the fallopian tubes, some arriving within fifteen minutes of their being deposited in the vagina, after passing through the cervix and uterus. Fertilisation occurs in the outer part of the tube near to the ovary.

An egg is released at ovulation, is picked up by the fallopian tube fimbriae, and the tube is responsible for the transport of the egg to the uterus which takes about four days. If the egg is fertilised in the tube, the first cell division into two new cells takes place within twelve to twenty hours, and successive divisions (doubling the number of cells each time) happen every fifteen hours. A bundle of sixty four cells reaches the uterus to implant six to seven days after ovulation, by which time the natural female hormones have prepared the uterine lining cells (endometrium). The embryo burrows into the endometrium and starts to form a placenta.

Increased progesterone production from the ovary (which ovulated) prepares the endometrium, but it is the pregnancy hormone (human chorionic gonadotrophin - hCG), produced by the developing embryo, which sustains pregnancy. Sensitive blood tests can detect early hCG once it is produced. Much more of this hormone is necessary for a urine test to become positive - so the urine pregnancy test will become positive later as levels rise. (A blood pregnancy test shows a positive result above 10 international units per litre and a urine test positive above 25iu/l)

It is not difficult to imagine how the delicately folded tube linings with specialised cells can become damaged by inflammation or infection, and the transportation of a developing embryo to the uterus may fail - but the embryo is still trying to develop and has a natural invasive nature, so it can implant in the tube to form a placenta and a resulting dangerous ectopic pregnancy - as happens in about 1% of UK pregnancies.

How do ectopic pregnancies happen?

Each month before a woman has her period:

• One of her ovaries produces an ovum.
• The ovum is drawn into one of the fine finger-like tubes called fimbriae, passes through it and is deposited at the end of the fallopian tube furthest away from her womb.
• The ovum makes its way, propelled by the cilia towards the womb. During the course of this journey it may encounter a man's sperm in which case it becomes fertile.
• If it is fertilised, the ovum implants itself into the special lining of the womb (renewed each month) called the endometrium and becomes a baby.
• If it is not fertilised then both ovum and lining are discharged in the menstrual flow, a fresh lining is prepared and a new ovum begins to ripen within the ovary.

So what can go wrong and cause an ectopic pregnancy?

• The fimbriae can fail to catch the ovum, in which case it may become fertilised outside her reproductive system altogether.
• More commonly, the fertilised ovum becomes caught while progressing down the fallopian tube In this case the baby continues to grow inside the tube where it can cause the tube to burst or otherwise severely damage it.

In either case a serious medical condition is likely to develop, requiring immediate attention (see the diagram below).

More information...

A more detailed account of ectopic pregnancies, aimed at doctors, medical students, nurses, midwives, and health ancillaries, presupposes some knowledge of medical matters on the part of the reader.

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