To form identical or monozygotic twins, one fertilised egg (ovum) splits and develops into two babies with exactly the same genetic information. To form fraternal or dizygotic twins, two eggs (ova) are fertilised by two sperm and produce two genetically unique children.
Identical, or monozygotic, twins occur when a single egg, fertilised by a single sperm, divides and makes two babies. These twins will be the same sex and share the same genes and physical features.
Identical twins (also called monozygotic twins) result from the fertilization of a single egg by a single sperm, with the fertilized egg then splitting into two. Identical twins share the same genomes and are always of the same sex.
Heteropaternal superfecundation is an extremely rare phenomenon that occurs when a second ova released during the same menstrual cycle is additionally fertilized by the sperm cells of a different man in separate sexual intercourse.
If more than one sperm fuses—a condition called polyspermy—multipolar or extra mitotic spindles are formed, resulting in faulty segregation of chromosomes during cell division; nondiploid cells are produced, and development usually stops. Two mechanisms can operate to ensure that only one sperm fertilizes the egg.
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.
Human eggs use chemical signals to attract sperm. New research from Stockholm University, Manchester University NHS Foundation Trust and The University of Manchester shows that eggs use these chemical signals to “choose” sperm. Different women's eggs attract different men's sperm – and not necessarily their partners..
Monozygotic (MZ) twins, also called identical twins, occur when a single egg cell is fertilized by a single sperm cell. The resulting zygote splits into two very early in development, leading to the formation of two separate embryos.
Superfecundation is the fertilization of two or more ova from the same cycle by sperm from separate acts of sexual intercourse, which can lead to twin babies from two separate biological fathers. The term superfecundation is derived from fecund, meaning the ability to produce offspring.
Sperm with many tails also have Macrocephaly, meaning that multiple tailed sperm come with the same problems as their big-headed associates: their chromosomal makeup is abnormal and therefore results in failed pregnancies and miscarriages when meeting with the egg.
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.
Clomiphene citrate (Clomid) and letrozole (Femara) are ovulation-stimulating medications. Both of these drugs are often given in IUI cycles and may help the body produce multiple eggs that may release at the same time. If two (or more) are fertilized and implant, twins are a possibility.
If the egg fertilized the couple were more likely to conceive a baby boy. Female sperm were found to be slower, fitter and would live longer than male sperm so making love earlier in the fertile window meant they would still be alive to fertilize the egg, and more likely to conceive a baby girl.
Several factors for causes of embryo splitting were suggested, including maternal age, prolonged embryo culture, ovarian stimulation, and zona pellucida (ZP) manipulation [6].
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.
Recently, researchers with the Institute of Life in Athens, Greece, announced that a healthy baby boy was born who basically had the DNA from three people. The child was born to a 32-year-old woman who had failed in four cycles of in vitro fertilization (IVF).
The biological phenomenon of giving birth to twins with two different biological fathers is called heteropaternal superfecundation.
It may be possible for stem cells from a male to be used to produce an egg, allowing for the child to have two biological fathers. Primordial germ cells (PGCs) are stem cells that give rise to sperm or egg cells.
Summary: Sperm in the first fraction of ejaculate are more numerous, move more and present better quality DNA than those lagging behind.
We often have heard that sperm from semen race towards the egg to enable fertilisation, with the fastest and fittest sperm winning the lottery. However, a study from 2020 highlighted that it isn't the sperm that rushes to claim its first position, it's in fact the egg that chooses which one gets to win.
The Sperm's Long Journey
Fast-swimming sperm can reach the egg in a half an hour, while others may take days. The sperm can live up to 48-72 hours. Only a few hundred will even come close to the egg because of the many natural barriers that exist in a woman's body.
Older women generally have a higher chance of conceiving twins. A 35 year old woman is about 4 times more likely to have fraternal twins than a 15 year old! That is because they are experiencing some hormonal changes as they near menopause. This could encourage their body to release more than one egg during ovulation.
They found that twins at first birth were linked to higher divorce rates at 13.7% compared to 12.7% for singletons. The link was statistically higher among moms attending college at 14.4% with twins vs. 10.5% with single births.
The mother gives an X chromosome to the child. The father may contribute an X or a Y. The chromosome from the father determines if the baby is born as male or female.
The concentration of sperm is what makes the semen cloudy and thick, so if your ejaculate is watery it is possible that you have a low sperm count. This doesn't mean you're infertile (so precautions to prevent pregnancy still need to be taken for couples that don't want to get pregnant).