Twins Identical and Fraternal

Twins Identical and Fraternal

Single fertilized egg splits - Two fertilized eggs develop


Types of Twin Pregnancy


Fraternal Twins

With fraternal twins (also known as 'non-identical twins'), the most common type, the mother contributes two eggs that each are fertilized in the womb by two different sperm cells from the father. The two eggs form two zygotes, and these twins are therefore also known as 'dizygotic' as well as 'biovular' twins.

Fraternal twins, like any other siblings, may look very similar, particularly given that they are the same age. However, fraternal twins may also look very different from each other. They may be a different sex or the same sex. The same holds true for brothers and sisters from the same parents, meaning that fraternal twins are simply brothers and/or sisters who happen to have the same age. They are genetically as similar as any ordinary siblings.

This type of twin is more common if there is a family history of twins, also more common for older mothers, with twinning rates doubling in mothers over the age of 35. They are also more likely to occur if fertility drugs are used to help you become pregnant.
Fraternal Twins

Identical Twins

With identical twins, one egg (zygote) from the mother is fertilized by one sperm from the father, and then very early in development the embryo splits and two fetuses grow. Spontaneous division of the zygote into two embryos is not considered to be a hereditary trait, but rather a spontaneous or random event.

If the zygote splits very early (in the first 2 days after fertilization) they may develop separate placentas (chorion) and separate sacs (amnion). These are called dichorionic, diamniotic (or 'di/di') twins.  While all fraternal twins are 'di/di', this occurs 20 - 30% of the time in identical twins.

Most of the time in identical twins the zygote will split after 2 days, resulting in a shared placenta, but two separate sacs. These are called monochorionic, diamniotic ('mono/di') twins.These twins are very similar genetically, and share a single afterbirth.

Very occasionally, twins will also share the same sac (fluid cavity). In about 1% of identical twins the splitting occurs late enough to result in both a shared placenta and a shared sac. These are called monochorionic, monoamniotic ('mono/mono') twins.

Conjoined twins

Finally, the zygote may split extremely late, resulting in conjoined twins. Popularly known as Siamese twins, conjoined twins occur in 1 in 100,000 births or 1 in 200 MZ twins. Conjoined twins can be identified via ultrasound in the early part of pregnancy. Apart from sharing placentas and membranes, conjoined twins also can also share body parts and organs. Occurrence of conjoined twins happens when the zygote divides late and doesn't split totally. The twins can be linked at any part of the body but both will be joined at the same place.

Semi Identical Twins
The 'semi-identical' twins are the result of two sperm cells fusing with a single egg, before becoming two embryos. An embryo created this way doesn't usually survive, but a few cases are known. Most of the time it goes totally unnoticed, and only shows up either if the two eggs have different genetic genders and the baby ends up with a mix of male and female gonads, or if it happens to be picked up on a genetic test that's done for other reasons (ex. paternity tests).

Twins: Symptoms  |  Prenatal Care  |  Check-ups  |  Complications



Common variations of twinning

The 3 most common variations are fraternal:
  • Male - female twins are the most common result, at about 40% of all twins born
  • Female fraternal twins (sometimes called 'sororal' twins)
  • Male fraternal twins
The last 2 are identical:
  • Female identical twins
  • Male identical twins (least common)


Symptoms of Twin pregnancy

Finding out that you are having twins can be both exciting and stressful. Twins happen in about one in 60 pregnancies, or as high as 1 in 4 with fertility drugs.

Women with twins may notice their pregnancy signs sooner or they may be more intense. This is because they have higher levels of hCG in their system. Other hormones increase during pregnancy as well, mainly progesterone and some oestrogen, which contribute to many of the early physical signs of pregnancy.

multiple pregnancyThe discovery you are expecting twins is often made quite early in pregnancy after you have a routine ultrasound scan. Two gestational sacs or fetus are seen rather than one.

A twin pregnancy is sometimes suspected if the womb feels larger than expected to the doctor or midwife who examines you. Later in pregnancy, the two separate babies may be felt abdominally during an examination, and again this will be confirmed by ultrasound scan.

Symptoms, signs and common problems of a single pregnancy, such as vomiting in early pregnancy, heartburn, shortness of breath, backache, ankle swelling, piles and varicose veins, may be worse in a twin pregnancy.


Twins Prenatal Care

Receiving regular prenatal care will help to ensure that you have a healthy pregnancy. However, there will be some differences in your prenatal health care if you are carrying multiples. Because you are carrying twins, you will be seen more often and you will be offered more testing. To be sure that any complications are detected and treated as soon as possible, you will be checked out more frequently than in a single pregnancy. Regular prenatal visits will allow the growth of your womb to be checked. To make sure that both babies are growing, several ultrasound scans will be performed throughout the pregnancy.

Pregnancy Nutrition for Twinstwins
Your babies are relying on you for all their vitamins and nutrients. When expecting twins, you may need to increase the amount of certain vitamins and minerals in your pregnancy diet in order to satisfy the needs of your babies. Your health care provider will probably recommend that you increase your intake of folic acid and iron in order to prevent neural tube defects and anemia. Water consumption is also essential with multiple pregnancies. Try to drink at least eight glasses of water a day.

Twin Pregnancy Weight Gain
Knowing how much weight to gain during the entire nine months can be difficult. Weight gain becomes even more complicated with multiple pregnancies. If you are expecting twins, it is generally suggested that you gain between 35 and 45 pounds over nine months, with the bulk of this weight gain occurring in the second and third trimesters. Be sure to speak with your health care provider about what a safe and healthy weight gain would look like for you.


Frequent Check-ups for Twin Pregnancies

To be sure that any complications are detected and treated as soon as possible, you will be checked out more frequently than in a single pregnancy.

ultrasoundRegular prenatal visits will allow the growth of your womb to be checked. To make sure that both babies are growing, several ultrasound scans will be performed throughout the pregnancy.

Eating a good balanced diet and getting extra rest are important. You will be given iron supplements and vitamins to stop anaemia developing.

When twins share either a sac or an afterbirth, you will be offered particularly careful ultrasound scans between 18 and 24 weeks, as a fast build-up of fluid can occur at this time. If the twins are in different sacs but share an afterbirth, a build-up of fluid is usually due to one twin getting a better blood supply than the other. This is called twin-to-twin transfusion syndrome. The condition must be treated, usually by withdrawing some of the fluid through a needle.

Your care provider may feel it is necessary to measure the length your cervix with ultrasound. This is done with an transvaginal scan, using a transducer which is specially shaped to fit inside the vagina. This measurement may be repeated during your pregnancy if there are any concerns about preterm labor or an incompetent cervix. A cervix is considered incompetent when it shortens considerably without contractions.

Cervical lengths of 30mm to 40mm are normal. If your cervix is less then 20mm, it will need close watching, bedrest and maybe a stitch known as a cerclage.


Twin Pregnancy Complications

You are also more likely to get more pregnancy complications with twin pregnancies. Carrying twins does put you in the higher risk category, but that does not mean you will develop complications. Most twin pregnancies do result in healthy babies and healthy mothers, too.
  • High blood pressure in pregnancy is about three times more common in twin pregnancies than in single pregnancies.

  • Anaemia is more common, when two babies make greater demands on your supplies of vitamins and iron. Eating healthily is a vital part of body care.

  • Too much fluid (hydramnios) around the baby can also be a problem. This, in turn, can also lead to premature labour.

  • Bleeding in late pregnancy, due either to a low-lying or partially detached placenta, is also more common in twin pregnancies.

Miscarriage with Twins
Unfortunately, your chances of having a miscarriage are higher when you are carrying more than one baby. That's mostly because the risk of chromosomal abnormalities is higher in a pregnancy with twins. And the chances that you'll lose one baby after 20 weeks or in the first month after birth are more than twice as high. A miscarriage is more likely if you are pregnant with identical twins. If you do miscarry one twin, the chances are low that something will go wrong with the surviving baby. The earlier the loss, the less likely you are to develop complications. In fact, the majority of remaining twins survive.

Vanishing Twin Syndrome
Another common potential problem in pregnancies with more than one baby is the so-called vanishing twin syndrome, and happens more often than you might think. A twin pregnancy can been diagnosed by an early scan in the first trimester. An ultrasound will detect twins from about six weeks when they are just 3mm long. Two tiny embryos can be seen but, about one in five of these will subsequently disappear before 12 weeks and only one grows to term.

You may spend the several weeks between the two scans imagining what it would be like to have twins and you have probably been getting excited about the idea, and may now feel sadness at the loss of your expectations. When one of a twin conception fails to survive at this early stage, the twin remaining continues to develop normally, and hormone levels stay high which prevents a miscarriage, though sometimes there is some vaginal bleeding.

As the remaining twin grows, its sac spreads to fill the uterus, and the contents of the other sac are absorbed. The vanishing lost twin is reabsorbed into the mother's body or miscarried with little or no warning and few or no symptoms, unlike a regular miscarriage, where you normally have cramping and bleeding. Why this happens is not completely understood.