Twins are rare and special, occurring in about 2% of all pregnancies. Of that number, 30% are identical twins. The other 70% are non-identical, or fraternal twins.
This animation will show you the differences between the development of a single baby, identical twins, and fraternal twins.
Starting with the single baby, let’s go back to the beginning, when fertilization occurs. Here you see that the egg cell is fertilized by a single sperm cell to form a zygote. Over the next few days, the fertilized egg cell divides over and over to form a structure composed of hundreds of cells called a blastocyst.
During the first week after fertilization, we can look inside the blastocyst and see the mass of cells that will form the embryo. The blastocyst will continue traveling toward the uterus where it will implant in the uterine lining, and grow into a single baby.
Now let’s watch the development of identical twins. Identical twins start out from a single fertilized egg cell, or zygote, which is why they’re also called monozygotic twins. Like the single baby we just saw, the egg cell is fertilized by a single sperm cell.
Unlike the single baby, this fertilized egg cell will split into two separate embryos, and grow into identical twins. This remarkable event takes place during the first week after fertilization, and can happen at several different times: at the two cell stage on day 2 at the early blastocyst stage on day 4 or in the late blastocyst stage on day 6.
The stage at which the egg cell splits determines how the twins will implant in the uterine lining, and whether or not they share an amnion, chorion, and placenta. Basically, the earlier the splitting occurs, the more independently the twins will develop in the uterus. So, a pair of identical twins that split during the two-cell stage will each develop its own amnion, chorion, and placenta.
Twins that split during the late blastocyst stage will share an amnion, chorion, and amniotic sac.
A common misconception about the conception of identical twins is that the trait for having them is passed on to future generations through the mother’s genes. But the truth is science doesn’t know the reason why identical twins occur. At this time, we can just say that they’re examples of a nine-month double miracle.
Now let’s take a look at the second type of twins. Non-identical, or fraternal, twins develop from two fertilized egg cells, or zygotes. Which is why they’re also called dizygotic twins. Unlike identical twins, however, fraternal twins are definitely influenced by the mother’s genes. Here’s why:
When the mother of fraternal twins ovulates, sometimes her ovaries release two egg cells for fertilization. Typically, only one egg cell is released during ovulation.
During conception, both of these egg cells become fertilized by two different sperm cells, which is why fraternal twins don’t look exactly alike. Sometimes they’re not even the same sex.
Here in the uterus, you can see that the twin embryos develop separately each having his or her own chorion, amnion, and placenta.
Review Date: 11/12/2021
Reviewed By: John D. Jacobson, MD, Department of Obstetrics and Gynecology, Loma Linda University School of Medicine, Loma Linda, CA. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.
Cell division
1:42
Cesarean section
3:47
Conception - general
1:46
Conception - pregnancy
3:20
Conception of identical twins
0:33
Early labor
0:52
Egg cell production
3:54
Egg production
4:02
Fetal ear development
1:13
Formation of twins
3:44
Human face formation
1:57
Intracytoplasmic sperm injection
0:49
Kids - How big is the baby?
1:15
Kids - How does the baby co...
0:42
Kids - Is it a girl or boy?
1:14
Kids - Umbilical cord
1:19
Kids - Where do babies come...
0:50
Ovulation
0:38
Placenta delivery
0:57
Placenta formation
Preeclampsia
0:18
Pregnancy
1:26
Sperm production
2:28
Sperm release pathway
1:53
The role of amniotic fluid
1:45
Twin-to-twin transfusion sy...
1:11
Ultrasound
2:55
Vaginal delivery
0:53
Vasectomy
0:32
Breast engorgement
2:48
C-section
2:14
Endometriosis
2:38
Hysterectomy
2:24
Infant formulas
2:37
Newborn jaundice
2:16
NICU consultants and suppor...
3:13
Pregnancy care
3:18
Storing breast milk
2:07
24-week fetus
Abnormal discharge from the...
Abnormal menstrual periods
Absence of menstruation (am...
Amniocentesis
Amniotic fluid
Anatomy of a normal placenta
Antibodies
Baby burping position
Bananas and nausea
Before and after vasectomy
Birth control methods
Blood cells
Blood test
Breast infection
Breastfeeding
Bulging fontanelles
Candida - fluorescent stain
Caput succedaneum
Childbirth
Chorionic villus sampling
Congenital hip dislocation
Congenital toxoplasmosis
Crying - excessive (0 to 6 ...
Delivery presentations
Developmental milestones
Early weeks of pregnancy
Ectopic pregnancy
Emergency Childbirth
Endocrine glands
Endometritis
Erythroblastosis fetalis - ...
Female breast
Female reproductive anatomy
Female reproductive anatomy...
Female urinary tract
Fetal blood testing
Fetal head molding
Fetus at 10 weeks
Fetus at 12 weeks
Fetus at 16 weeks
Fetus at 26 to 30 weeks
Fetus at 3.5 weeks
Fetus at 30 to 32 weeks
Fetus at 7.5 weeks
Fetus at 8.5 weeks
First trimester of pregnancy
Folic acid
Folic acid benefits
Folic acid source
Follicle development
Fontanelles
Foreskin
Genetic counseling and pren...
Gestational ages
Gestational diabetes
Gonadotropins
Head circumference
Heat rash
Height/weight chart
Hormonal effects in newborns
Hormone-based contraceptives
Humidifiers and health
Infant blood sample
Infant care following delivery
Infant diaphragmatic hernia
Infant heat rash
Infant intestines
Infant jaundice
Infantile reflexes
Influenza vaccines
Intraductal papilloma
Intrauterine device
Intrauterine transfusion
Jaundiced infant
Large fontanelles
Large fontanelles (lateral view)
Macrosomia
Male reproductive anatomy
Male urinary tract
Mammary gland
Meconium
Morning sickness
Moro reflex
Newborn head molding
Newborn test
Normal female breast anatomy
Normal uterine anatomy (cut...
Ovarian cyst
Ovarian hypofunction
Overproductive ovaries
Pelvic adhesions
Pelvic laparoscopy
Placenta
Placenta previa
Polyhydramnios
Pregnancy test
Primary amenorrhea
Primary infertility
Secondary amenorrhea
Secondary infection
Side sectional view of fema...
Single palmar crease
Skull of a newborn
Slit-lamp exam
Sperm
Stein-Leventhal syndrome
Sunken fontanelles (superio...
The cervical cap
The diaphragm
The female condom
The male condom
Tobacco health risks
Transvaginal ultrasound
Tubal ligation
Ultrasound in pregnancy
Ultrasound, color - normal ...
Ultrasound, normal fetus - ...
Ultrasound, normal fetus - face
Ultrasound, normal fetus - foot
Ultrasound, normal placenta...
Ultrasound, normal relaxed ...
Umbilical cord healing
Uterus
Vaginal bleeding during pre...
Vaginal ring
Well baby visits
Yeast infections
CPR - infant - series
Exchange transfusion - series
Rh incompatibility - series