My science textbook says that a mature human female ovary releases one egg each month. So, how are twins born?

Wouldn't twins need two eggs? If two eggs are released, how does that happen?





Also, how do you explain triplets, quadruplets and so on?

My science textbook says that a mature human female ovary releases one egg each month. So, how are twins born?
On average one a month is released.





Or so it is assumed.





It's kinda hard to be digging into such a soft place with metal tweezers and expect to get much cooperation every day.





Sometimes more than one is released, thus more than one egg is fertilized. Fraternal kids are created.





Or, one egg fertilized splits into two or more embryoes and thus multiple identical kids are created.





Or, sometimes the mechanism screws up, and one ends up with parasitic twins.





Fertility drugs and pollutants have an effect in which multiple eggs can be released.
Reply:Identical twin (trips, etc) result from only one egg and in the earliest stages of development the embryo splits.





In the case of fraternal sets, either both ovaries release an egg or one releases more than one. This can be caused by many factors including the use of fertility drugs.
Reply:A female has two ovaries, so it's possible for two eggs to be released at once. This is what happens with Fraternal twins. Two eggs are fertilized by two sperm (one per egg), which then implant in the uterine wall in two different places. Since they are from two eggs and two sperm, they are no different than any other brothers or sisters (or brother and sister - since they are from two different sperm, they don't have to be the same gender). They are functionally two separate pregnancies that just happen to occur at the same time.





Sometimes more than one egg will be released per ovary. One egg per month is the 'norm' but some women naturally differ. Sometimes it's the result of fertility drugs which artificially boost the rate, sometimes it's just natural variation between people.





Identical twins arise from *one* egg fertilized by *one* sperm. The fertilized cell develops normally for a few divisions, but then splits in two. It then develops normally, in parallel with it's other half (since it is so early in development, it can compensate for losing half of its cells without any problems). The result is two genetically identical embryos that share one amniotic sac and one placenta.





Sometimes the embryo can split more than once, leading to multiple identical siblings (not necessarily in multiples of two - if one embryo split, then only one of the resultant twins split, you could end up with identical triplets). It's a slightly more complex process, though, so I think identical triplets (and above) are more rare than, say, fraternal triplets.
Reply:Identical twins are when the cells of a single fertilized egg divide completely in two, forming two identical sets of cells that become fetuses then babies. Identical multiples (very rare) are the result of even further splitting of the single fertilized egg.





Fraternal twins are the result of 2 sperm fertilizing one egg and it's splitting into two separate non identical sets of cells. Sometimes women's ovaries do release more than one egg and they all get fertilized.





Larger multiple births (those that occur naturally are very rare. Most of them these days are the result of fertility drugs that force the ovaries to release more than one egg each month.
Reply:I have twins! There are two ways multiples can occur. The first, some woman (me!) release MORE THAN ONE egg per month. Each of those eggs gets fertlized by different sperm, and that causes a Multiple Birth. And second, an egg can SPLIT (after it's been fertilized)! So, in the case of a woman who only produces one egg per month ... her egg has SPLIT (it breaks in two, or three, or four, etc.) and that causes a Multiple Birth. I'm not sure why your textbook is WRONG!
Reply:Monozygotic twins (identical twins, in common parlance) =


one fertilized ovum dividing while the cells still have the potential to develop into independent organisms (2 babies).


These are true twins, genetically identical (clones).





Sometimes multiple ova are released. Dizygotic twins (fraternal twins) = two ova fertilized by two sperm which develop into babies. Dizygotic twins are no more related than siblings born years apart. They simply happen to develop in utero at the same time.
Reply:Wow. I thought the Florida education system was bad.





Brandon
Reply:The human female have tow ovaries, left and right. Each will release an ovum at a time. If both ova were fertilized the result will be in-identical twins. The other case of twinning is that a single ovum is fertilized, divided into tow embryos each embryo will give a child resulting in identical twins. Triples may result from one ova giving tow embryos and the other giving one or both ova giving tow but one embryo dies. Quadruplets is tow ova giving tow differed embryos or one ova giving tow embryos and the tow are further divided into four embryos.
Reply:Twins are results of two sperms entering an ovum, the female body does release only 1 egg or ovum per month