What is an ectopic pregnancy?

Tubal ectopic pregnancy - ampullary, isthmic and fimbrial

The Fallopian tubes are small, hollow muscular tubes, each about ten centimetres long. The outer half lies next to, but not attached to, its ovary. The Fallopian tubes have a delicate mucous membrane lining inside the tube, thrown up into folds, which almost fill each tube. The diameter and the number of folds increase as the Fallopian tube nears the ovary and forms the fimbriae – tiny finger-like projections that move and create a suction effect to draw the egg to the Fallopian tube.

In the lining of the Fallopian tubes, half the cells are mucus-secreting and half have cilia – tiny hair-like projections which waft gently to propel these secretions towards the uterus. The Fallopian tube has a natural peristaltic action (contraction and relaxation to create a pumping effect) which assists the movement of mucus.

An egg, released at ovulation, is picked up by the Fallopian tube fimbriae, and the Fallopian tube is responsible for the transport of the fertilised egg to the cavity of the uterus which takes about four days. If a pregnancy is as expected, 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.

Where pregnancies do not reach the uterus, it is not difficult to imagine how the delicately folded Fallopian tube linings with specialised cells can become damaged by inflammation or infection, and/or the transportation of a developing embryo to the uterus may be slow for no obvious reason. In the meantime, the embryo is still trying to develop and has a natural invasive nature, so it can implant in the Fallopian tube or fimbriae to form a placenta, resulting in a potentially dangerous ectopic pregnancy.

Interstitial pregnancy

An interstitial pregnancy is a less common type of ectopic pregnancy that occurs when the fertilised egg implants in the part of the Fallopian tube as it crosses the wall of the uterus.

Pregnancies of this kind are difficult to diagnose as they may appear to be in the uterus on a scan or may be difficult to see on a scan very early on. They are particularly dangerous if they are growing, as they can progress further and tend to rupture later, having the potential to damage both the wall of the uterus and the Fallopian tube.

Some doctors call interstitial pregnancies ‘cornual’, which is confusing, so the term ‘interstitial’ is preferred.

Caesarean scar pregnancy

Caesarean scar ectopic pregnancies are when the fertilised egg implants into the gap in the muscle of the uterus caused by a previous Caesarean section. The pregnancy may then grow out of the uterus or onto the cervix and cause torrential internal or vaginal bleeding.

In some pregnancies, the placenta develops so that only part of it is within the scar and these pregnancies may proceed to the delivery of a live baby, but with risk of significant bleeding from the mother and a hysterectomy at the time of delivery.

The treatment of Caesarean scar pregnancies is potentially difficult, so management has to be individualised on a ‘case-by-case’ basis.

Research indicates that this kind of ectopic pregnancy appears to be increasing, possibly due to the impact of elective caesarean section delivery which was much less common 10 years ago than today. However, despite appearing to increase in incidence, they are still relatively uncommon and so some hospitals see very few of them. Accordingly, there are moves to focus care in these cases to regional units with experience of these cases both to optimise care and to collate information to make sure there is a better evidence base to inform treatment decisions in the future.

Cervical pregnancy

The cervix is highly vascular (lots of blood vessels) and, when the pregnancy separates from the cervix, blood transfusions are usually essential.

Scarring of the passage through the cervix due to gynaecological procedures has been identified as one of the leading risk factors for a cervical ectopic pregnancy.

Rudimentary horn pregnancy

A rudimentary horn pregnancy is rare and can only occur if there is a uterus that has not formed as expected. When it is being formed usually, the uterus develops from two structures that join together in the middle. Sometimes these two structures do not develop equally and join, and this leaves one (unicornuate) side which is in contact with the vagina and another smaller side (rudimentary horn) that is not in contact with the cervix and vagina. Both sides have their own Fallopian tube. Sperm can reach deep inside via the one-sided uterus that is in contact with the vagina, but the opposite Fallopian tube may pick up the fertilised egg and transport it into the rudimentary horn.

Because the rudimentary horn often has a thick muscular wall, these pregnancies may advance into the second or even third trimester before they rupture. The thick wall also makes them more difficult to diagnose, as the rudimentary horn may be assumed to be a normal uterus during an ultrasound scan.

A pregnancy in the uterine portion of the Fallopian tube of a uterus that forms as expected is called an interstitial pregnancy. This should not be confused with a rudimentary horn or cornual pregnancy.

Ovarian pregnancy

Ovarian ectopic pregnancies can be difficult to diagnose as they can look very similar to a tubal ectopic pregnancy that is stuck to the ovary or a ‘corpus luteum’, which is the place that the egg was released from. This can mean that ovarian pregnancies are often not diagnosed until surgery.

The ovary is a highly vascular structure. An ectopic pregnancy located on or in the ovary will usually require surgery involving either the partial or complete removal of the ovary due to bleeding. If the ovary is partially removed, it may recover and continue to produce eggs as before.

However, even if it no longer produces eggs or is removed completely, the other ovary is usually perfectly capable of producing an egg every cycle, enabling the ability to conceive naturally in the future.

Intramural pregnancy

This is a term that means ‘in the wall’ and refers to a pregnancy that implants outside the cavity of the uterus, but within its muscular wall.

These pregnancies are thought to occur when the uterus has been scarred by previous surgery or a condition called adenomyosis. Again, they can be difficult to diagnose as it can be hard to see the cavity as separate to the pregnancy.

Abdominal pregnancy

Abdominal pregnancies, in most instances, are thought to have begun in the Fallopian tube and then separated from the wall of the fallopian tube, floating into the abdominal cavity to then reattach to one of the structures in the abdomen.

The pregnancy can progress and may go undetected until many weeks into the pregnancy. There are some accounts of abdominal pregnancies surviving to be delivered with an abdominal operation but these are incredibly rare.

Heterotopic pregnancy

A heterotopic pregnancy is a type of twin pregnancy where there is one normally sited pregnancy and one ectopic pregnancy. The majority of co-existing ectopic pregnancies will be in a Fallopian tube. In many cases, it is possible for the normally sited pregnancy to continue, even if surgery is needed for the tubal ectopic pregnancy.