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Twinning - Causes, Types, Diagnosis, and Risks

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The act of producing two children in one birth is known as twinning. Read the article to know the cause and management.

Medically reviewed by

Dr. Monica Mathur

Published At March 10, 2023
Reviewed AtMarch 10, 2023

What Is Twinning?

Giving birth to more than one child is known as multiple births, and when the number is two, it is known as twinning. Multiple pregnancies with three (triplets), four (quadruplets), or more babies are rare but not impossible.

Natural twinning used to be around three percent of the total live births, but this number has increased in recent years due to assisted reproductive technology (ART) and geriatric pregnancy (advanced maternal age). Pregnancy-related complications are exaggerated in twin births compared to normal birth, except for fetal macrosomia (a larger newborn) and post-term pregnancy (prolonged pregnancy).

What Causes Twinning?

There are two different ways of twinning:

  1. Splitting of the fertilized egg (monozygotic twins) before it implants in the uterus; the twins born this way are known to be identical.

  2. Fertilization of two (dizygotic twins) or more eggs by different sperms simultaneously, resulting in fraternal twins.

Dizygotic twinning is commonly seen in geriatric and ART-assisted pregnancies, and this is because of the way the twins are conceived. In both pregnancies, women face ovulation complications (infertility issues) and are then prescribed ovulation induction drugs to increase the chances of pregnancy.

These drugs stimulate egg production, and the result is often more than one egg. These eggs are then fertilized through sexual intercourse, intrauterine insemination, or other fertility treatments, forming dizygotic twins.

On the other hand, monozygotic twins are thought to be genetic, a hereditary trait passed down from the material side of the family. However, there have been cases where in-vitro fertilization (IVF) has caused embryo cleavage (splitting), leading to two babies from the same egg. The risk for conjoined twins increases if the splitting happens after 14 days after fertilization.

How Are the Sex of Twins Determined?

Monozygotic twins or identical twins are always of the same sex because they start as one fertilized egg and then split after fertilization, so whatever chromosome combination (XX or XY) was assigned at fertilization defines the sex of both babies. Dizygotic or fraternal twins can be different or same-sex; it is determined by the sperm cell that fertilizes the egg during fertilization.

The egg and the sperm are haploid (one set of chromosomes), the former always carries the X chromosome, but the latter can carry X or Y chromosome. During fertilization, the eggs can be fertilized with either of the sperm leading to three possible combinations:

  1. A boy (XY) and a girl (XX).

  2. A boy (XY) and a boy (XY).

  3. A girl (XX) and a girl (XX).

What Are the Signs of Twinning?

Women carrying twins experience the same pregnancy symptoms as a single pregnancy but at an increased intensity; some of them are:

  1. Extreme morning sickness (nausea and vomiting).

  2. Women carrying twins gain weight rapidly; it can be anywhere between 11 to 25 kilograms, the lower end is appropriate for obese women, and the upper end is suitable for women of normal weight.

  3. Increased HCG (human chorionic gonadotropin) levels, a hormone produced during pregnancy, indicate twin pregnancy.

  4. Increased amounts of alpha-fetoprotein in the expecting mother's blood are also indicative of twin pregnancy. It is a protein that the baby makes while developing in the womb; some of this usually passes into the expecting mother’s blood which shows up in the test. In twinning, this protein is doubled due to the presence of two babies, and it is also more in cases of babies with birth and genetic defects.

How Is Twinning Diagnosed?

  1. Ultrasound is the best way to confirm twinning. Determining twin-specific complications like twin-to-twin transfusion syndrome (TTS), fetal growth restriction (FGR), etc are also useful.

  2. Tests like doppler velocimetry, non-invasive prenatal testing, and amniotic fluid volume are also helpful in identifying risky twin pregnancies, but these cannot be used to confirm twins.

  3. The next step is to determine if the twins are identical or fraternal; this is usually done after birth with the help of a zygosity test.

What Are the Complications of Twinning?

Pregnancy itself comes with a few risks, which get amplified with twinning. This does not automatically mean that every expecting mother carrying multiple pregnancies will face the same complications; as long as the babies are growing appropriately without birth defects, there is no reason to assume otherwise. Some of the commonly seen twinning complications are:

1. Preeclampsia - Also known as gestational hypertension, it is a condition where the blood pressure of the expecting mother is abnormally high (greater than 140/90 mmHg) along with proteinuria (high levels of protein in the urine).

This is extremely dangerous for the mother and the babies, often leading to preterm delivery with health complications like low birth weight and respiratory issues.

2. Premature Birth - The ideal gestation period for women with twins is around 37 weeks; babies born during this time are healthy and of the appropriate weight. However, this might be shortened in a few cases due to underlying medical conditions, leading to early delivery.

3. Twin-To-Twin Transfusion Syndrome (TTTS) - It is a prenatal condition in which twins share unequal amounts of the placenta's blood supply resulting in two babies growing at different rates. If not detected early, this can lead to prenatal birth or prenatal loss.

4. Twin Anemia Polycythemia Sequence - Specifically seen in identical twins, it is characterized by highly discordant intertwin hemoglobin difference with a normal amniotic fluid volume. Complications are similar to TTTS.

5. Fetal Growth Restriction - It is another prenatal condition commonly seen in identical twins and happens due to uneven distribution of the placenta. The placenta provides the necessary nutrition to the growing babies, imbalance in this distribution will lead to the malnourishment of one of the twins.

A few other complications of twinning are gestational diabetes and placental abruption, which can also be seen in singleton pregnancies.

How To Stay Healthy During Multiple Pregnancies?

The healthcare provider will advise the necessary precautions and steps for a healthy pregnancy, similar to a normal pregnancy, with a few exceptions. They are:

  1. An ultrasound every two weeks, beginning from the sixteenth week, to observe the growth and detect any abnormalities.

  2. Nutrition and prenatal supplements are extremely important during pregnancy. The rule of thumb is to consume 300 calories per baby, which doubles in twinning. This is an estimate and should always be consulted with a healthcare provider before following up.

  3. Although vaginal delivery is possible, it is important to have a birth plan to avoid complications.

  4. Expecting mothers are encouraged to exercise, which relieves the discomfort often seen during pregnancy. Except for a few cases, low-impact exercises like swimming, walking, and prenatal yoga can help maintain the health of the mother and the baby.


With the advent of advanced fertility treatments, twinning, and multiple pregnancies have become more common than they used to be. The focus should be on preventing preterm birth and early diagnosis of genetic defects, which are relatively common in twins. Communicating with the healthcare provider can avoid many misconceptions regarding the birth and the baby's health.

Dr. Monica Mathur
Dr. Monica Mathur

Obstetrics and Gynecology


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