Taken from: "When you're expecting Twins, Triplets, or Quads", by Dr. Barbara Luke and Tamara Eberlein. Chapter 1, pages 6-8
(Dr. Luke, Sc.D., M.P.H., R.D., a professor of obstetrics and gynecology, directs a successful prenatal program for mothers of multiples at the University of Michigan in Ann Arbor. She is also a practicing dietitian and the author of "Every Pregnant Woman's Guide to Preventing Premature Birth", and "Good Bones: The Complete Guide to Building and Maintaining the Healthiest Bones".)
Conception of Identicals.
~Every month during a woman's childbearing years, her ovaries release a mature egg. It's called OVULATION.
~The egg travels down the fallopian tubes, where FERTILIZATION (or CONCEPTION) occurs if the egg unites with the father's sperm. During FERTILIZATION the genetic material from both father an mother combine. Then the egg begins to divide.
~Within a week of FERTILIZATION, the dividing egg (which is now called a ZYGOTE), embeds itself into the lining of the uterus.
~The egg begins to separate into inner and outer layers.
The outer layer grows to form the PLACENTA.The inner layer becomes the EMBRYO, which develops into the baby itself.
~By the second week of life, the EMBRYO is firmly implanted in the lining of the uterus.
The conception and development of multiples is similar to that of a singleton baby, but with a few small differences:
**Identical twins begin the same way as a singleton--WITH ONE EGG and ONE SPERM.**
~The ZYGOTE (which is what the dividing egg is called) then divides yet again, producing two separate and identical ZYGOTES.
~Identical twins are also called MONOZYGOTIC, meaning they developed from a single (mono) egg.
The timing of the additional division of the egg determines the structure of the fetal membranes:
~When the split happens within three days of conception, while the single, original ZYGOTE is still travelling down the fallopian tube, these identical twins will have two separate placentas...two outer membranes (called CHORIONS), and two inner membranes (called AMNIONS). This is called a "diamniotic, dichorionic" twin pregnancy.
~When the split happens 4-7 days after conception, the twins will have separate AMNIONS, but will share one CHORION, and the placentas will be fused. This is called a "diamniotic, monochorionic" twin pregnancy.
~If the split happens after the 8th day following conception, the identical twins will share a placenta, chorion, and amnion. It is called a "monoamniotic, monochorionic" twin pregnancy.
Other Identical Twin Facts:
~Approximately 1/3 of all twin pairs are identical.
~Because identical twins share 100% of their genes, they are always of the same sex, and have the same basic physical features, such as hair/eye color.
~Some pairs are mirror-image--one is right handed, one is left handed; their hair whirls can be reversed; they may have matching birthmarks on opposite sides of their bodies...
~They can vary significantly in size at birth because one may have received a disproportionate share of nourishment in utero, by adulthood, height usually differs by less than two inches.
~Identicals have very similar IQ's, and often share many personality traits.
**So the whole "the sperm divides the egg" myth is nothing more than that. A myth. The sperm has zero to do with the egg dividing...only the mother does In fact, sperm has NOTHING to do with determining ANY twin type. (In the case of fraternals, women alone can determine how many eggs are going to be dropped. All the fraternals in the world on her spouse's side of the family isn't going to change that fact.) Sperm ONLY FERTILIZE an egg, NOTHING MORE. The egg just happens to divide one extra time with identicals, which happens AFTER conception...AFTER the sperm has completed it's mission.
"....monozygotic (IDENTICAL) twinning is completely random..." (page 12)
Hopefully this will be the end of that insane myth. Every time I hear it said, I picture the sperm with tiny saws, cutting the egg in half to make two babies instead of one. Or a sperm with muscles so big that when it runs into the egg, it just immediately bursts into two from the sheer force of impact. I hope in my lifetime that this knowledge will become common. (I also hope that one day I'll stop having people ask me if my b/g twins are identical.)
Spread the message. End the madness.