Multiple pregnancy usually happens when more than one egg is fertilized. It also can happen when one egg is fertilized and then splits into 2 or more embryos that grow into 2 or more babies. When one fertilized egg splits into 2, the babies are called identical twins.
Factors that increase the chance of twins include: consuming high amounts of dairy foods, being over the age of 30, and conceiving while breastfeeding. Many fertility drugs including Clomid, Gonal-F, and Follistim also increase the odds of a twin pregnancy.
Fraternal twins result when two different eggs are fertilized, each by a different sperm cell. This can happen when a woman produces several eggs (most often two) at the same time. This is called hyperovulation. Fraternal twins don't share the same DNA.
Although certain factors increase the chance of having twins, there is no way to improve the odds of having twins naturally. A woman will usually find out that she is pregnant with twins from an ultrasound early on in the pregnancy.
However, for a given pregnancy, only the mother's genetics matter. Fraternal twins happen when two eggs are simultaneously fertilized instead of just one. A father's genes can't make a woman release two eggs.
Everyone has the same chance of having identical twins: about 1 in 250. Identical twins do not run in families. But there are some factors that make having non-identical twins more likely: non-identical twins are more common in some ethnic groups, with the highest rate among Nigerians and the lowest among Japanese.
Early signs of a twin pregnancy include severe morning sickness, quick weight gain, and more breast tenderness. You may also notice an increased appetite or extreme fatigue. Plus, those who have a twin pregnancy may begin showing sooner.
Foods naturally rich in folic acid like avocado, spinach, broccoli, liver, and legumes also have the tendency to increase one's chances of conceiving twins. According to a study, consuming extra folic acid while trying to get pregnant can raise the chance of giving birth to twins slightly higher.
All pregnancies start when a sperm fertilises an egg. This fertilised egg is called a zygote. Sometimes a woman's ovaries release 2 eggs, and a separate sperm fertilises each egg. This forms twins.
Boys' and girls' genitals develop along the same path with no outward sign of gender until about nine weeks. It's at that point that the genital tubercle begins to develop into a penis or clitoris. However, it's not until 14 or 15 weeks that you can clearly begin to see the differentiated genitalia.
Several factors for causes of embryo splitting were suggested, including maternal age, prolonged embryo culture, ovarian stimulation, and zona pellucida (ZP) manipulation [6].
Many people believe twins skip a generation, but that's just a myth. The idea that twins skip generations likely comes from the fact that the genetic factors contributing to twins only come from the gestational parent's side.
Is there a third type? Traditionally, the science around twins has taught that identical and fraternal are the only two types. But a third type might exist, called polar body or half-identical twins.
Monoamniotic-monochorionic Twins
This is the rarest type of twin, and it means a riskier pregnancy as the babies can get tangled in their own umbilical cords.
Identical twins are almost always the same sex, although there are some rare exceptions to this rule. Because identical twins occur when a single fertilized egg splits and forms two embryos, each embryo has the same chromosomes (usually, XX for girls or XY for boys). Remember that sex and gender are not the same.
Fraternal Twins Boy and Girl
Since conception happened for each egg, each egg carries the sex chromosome of the father. A girl has XX chromosomes while a boy has XY chromosomes. A boy-girl combination is the most common form of these kind of twins.
Identical twins will always have the same blood type because they were created from the same fertilized egg (fraternal twins can have different blood types — again, providing the parents do — because they are created by two fertilized eggs).
Occasionally, two sperm are known to fertilize a single egg; this 'double fertilization' is thought to happen in about 1% of human conceptions. An embryo created this way doesn't usually survive, but a few cases are known to have made it — these children are chimaeras of cells with X and Y chromosomes.
In 99.9% of cases boy/girl twins are non-identical. However, in some extremely rare cases resulting from a genetic mutation, identical twins from an egg and sperm which began as male (XY) can develop into a male / female pair.
When are identical twins actually formed? Though all identical twins are formed very early in pregnancy, there is some variation in exactly how early the fertilised egg splits into two. This can happen at the two cell stage on Day 2, the early blastocyst stage on Day 4, or in the late blastocyst stage on Day 6.
Girls undergo puberty earlier than boys by about 1-2 years, and generally finish the stages of puberty quicker than males due to their differences in biology.
They found that the most fertile months for conceiving a boy were from September to November, while the lowest fertile period was from March to May.
The lungs are the last major organ to finish developing. When fully mature, they produce a chemical that affects the hormones in your body.
Research shows girls kick as often as boys. Babies who kick a lot in the womb are also more active after birth. Some mothers have more trouble feeling the kicks than others. If the placenta is on the front side of the womb, or if you are overweight, you will feel the kicks less.