A pregnancy with more than one foetus developing in the uterus is referred to as a multiple pregnancy - two embryos (twins), three embryos (triplets) and so on. The occurrence of twins is approximately 1: 80, but the number of multiple pregnancies is currently increasing considerably due to artificial insemination methods. Genetics can play a role in the likelihood of having a multiple pregnancy, however only the mother’s genetics play a role (more likely if there is a history of twins in the mother’s family). Race also plays a role, as black or african-american ethnicities tend to have the highest occurrence of twins, whereas Asians tend to have the least. A woman who has already given birth to twins is 10 times more likely to have a multiple pregnancy.
What are the causes of multiple pregnancies?
A multiple pregnancy occurs when one or more eggs are fertilised by one or more sperm. When a single fertilized egg splits, it results in multiple identical embryos, creating identical twins. These types of twins are of the same sex, are strikingly similar in physical appearance and have very similar mental and physical capabilities. Fraternal twins develop when more than one egg is released during the menstrual cycle and each is fertilised by one sperm. They may or may not be of the same sex and are not as physically and mentally similar as identical twins.
How are identical twins further classified?
Identical twins can be split into two categories depending on when the egg splits which determines whether they share the placenta and the foetal membrane. If the embryo divides in very early stages of pregnancy (around the third day after fertilisation), the two embryos have their own foetal membrane and their own placenta from which they are nourished, however this is not very common. Much more common is when the embryo splits later (4-8 days after fertilisation) and the embryos develop with a shared placenta, however they each have their own foetal membranes. If the embryo divides past day 9 post-fertilisation, the embryos share both a placenta and a foetal membrane and develop in one cavity in the uterus.
What are Siamese twins?
Siamese twins, also known as conjoined twins, are an extremely rare phenomenon that comes about from the incomplete division of an identical embryo.
What type of twins is the most common?
The most commonly occurring twins are fraternal twins. The prospect of having fraternal twins is 3-4x more likely than having identical twins.
What causes fraternal twins?
To form fraternal or dizygotic twins, two eggs (ova) are fertilised by two sperm and produce two genetically unique children. The embryos are formed from two separate cells, so they each have their own placenta and foetal membranes.
How a multiple pregnancy is identified
From the 6th week of pregnancy, therefore around the 25th-30th day after fertilisation, a multiple pregnancy can be identified via an ultrasound. Distinguishing between identical or fraternal twins is possible towards the end of the first trimester. However, multiple pregnancies can also be detected by examining hormonal levels, especially the pregnancy hormone hCG, the levels of which will be higher the more foetuses are present. During a pelvic exam, the doctor will notice an enlarged uterus, and two heart beats will be observable during an ultrasound exam.
How does a twin pregnancy differ from a single pregnancy?
A woman’s uterus and her general anatomy are adapted to the development of only one foetus, and therefore a multiple pregnancy has a greater burden on the woman's physiology with a greater risk of complications for both the mother and the foetus. Extreme morning sickness, miscarriage, premature birth, preeclampsia (a pregnancy condition associated with high blood pressure, protein in the urine, and general swelling of body parts- oedema) are more common in a multiple pregnancy. An excessively enlarged uterus can create pressure on the diaphragm, which can lead to subsequent digestive and respiratory problems, as well as increased stress on the blood vessels in the pelvis which may result in swelling of the lower extremities, external genitalia, and may also cause varicose veins. There is also more stress on the spine which can lead to back pain.
What is the vanishing twin syndrome?
Vanishing twin syndrome is a type of miscarriage, usually in the first 12 weeks of pregnancy. It is most often caused by the poor implantation of one of the foetuses. This results in a spontaneous miscarriage which is manifested by spotting/bleeding and abdominal pain. Sometimes, the miscarriage does not occur and the foetus that stopped growing remains in the uterus where it gradually shrinks and compresses (mummifies). The existing foetus continues to develop and usually has no complications.
Why are twin pregnancies more at risk of preterm labour?
The main cause of premature birth in a multiple pregnancy is the excessive stretching of the uterus which leads to its irritation and consequently a shortening of the birth canal and opening of the cervix. This can be controlled by regular cervical checks and early hospitalisation. Twins are generally born in the 37th week of gestation, and triplets in the 33th week.
What other complications are associated with multiple pregnancies?
Asymmetrical growth of the foetuses is a serious complication in multiple pregnancies. It is quite usual for twins to be of slightly different sizes, however in some cases this difference may become severe. This complication particularly affects identical twins who share the same placenta- the blood vessels in the placenta may create abnormal connections which can lead to one of the foetuses’ receiving more blood and nutrients at the expense of the other. In this scenario, both foetuses are at risk, as the one that doesn’t receive enough will deteriorate, and the other foetus will receive excessive blood and nutrition and will struggle to process the volume. This rare condition is called twin-to-twin transfusion, and the manifestations aren’t usually apparent until the 20th-30th week of gestation. Consequently, the ‘donor’ twin has a much lower weight than expected, has little amniotic fluid and its general well-being is deteriorating. On the other hand, the ‘recipient’ foetus is larger, has more amniotic fluid, but it’s condition usually also starts deteriorating. The smaller, ‘donor’ twin may consequently die. Frequent ultrasound checks are needed to detect this condition, and if it is detected early enough, a laser surgery on the placenta can be performed to partially separate the vascular connections and relieve the imbalance in amniotic fluid between the foetuses. If the condition of both the foetuses deteriorates, the pregnancy must be terminated.
Uneven foetus growth can also occur in fraternal twins. The most common cause of this condition is the inadequacy of one of the foetus’s placenta, which causes faltered growth in that foetus. Regular monitoring of placental function by ultrasound and monitoring of foetal heart rate by cardiotocography is required.
A rare variation of the presentation of identical twins is when they have one common placenta and are located in one sac (monochorionic monoamniotic twins). In this case there is a risk of the tangling of the umbilical cords which puts the foetuses at an increased risk of death. Such twins must be closely monitored in the hospital and the pregnancy must be terminated in good time- they are usually born very prematurely.
Is it possible to have a natural birth with multiple pregnancies?
There is a large variety of positions the foetuses can be in while in the uterus, and the decision on the birthing method depends on the length of the pregnancy, foetal placement, and the obstetricians experience. If the first foetus starts pushing on the birth canal with its head, it is generally possible to have a natural vaginal birth. In other variations of first foetal descent and in triplets or more, it is recommended to give birth via caesarean section.
A rare complication of natural birth of twins the so-called locked twins, where two foetuses become interlocked during presentation before birth. They can become wedged by their chins during labour, and in this case, it is necessary to perform a C-section.
What are the advantages and complications of the natural birth of twins?
The foetuses in a multiple pregnancy tend to be naturally much smaller than in a single pregnancy, so the disparity between the size of the foetus’s head and the mother’s pelvis is quite rare. On the other hand, the uterus function may be impaired during and after delivery due to its excessive expansion during pregnancy. It is sometimes necessary to strengthen the contractions of the uterus for this reason, as after delivery, an under-contracted uterus may lead to more postpartum bleeding. During natural birth, the second foetus is more endangered, because after the birth of the first foetus, the uterus partially contracts, leading to a deterioration in blood and nutrient flow to the second foetus which may decrease its oxygen supply. Therefore, the birth of the second foetus is always intensively monitored, and a very active approach is taken to its delivery.