What do fraternal twins develop from

Meconium — made up of digestive secretions, sloughed cells, and swallowed amniotic fluid — is collecting in their bowels and will be their first bowel movement after birth. Things are starting to get a little crowded for them now as they grow bigger. And you may notice them becoming more active now. Your mid-pregnancy ultrasound between 18 and 22 weeks will identify the sex of your babies, if you like. The sizes of the babies and their heartbeats will be measured, along with the amount of amniotic fluid.

Your babies' skin is red and wrinkled now, no matter what color it will be after birth, and their eyebrows are growing in.

They begin to store fat, and their lower airways are developing. They're growing more hair on their heads and taste buds on their tongues. Their bone marrow has started making blood cells.

Testicles will descend into the scrotum of boy babies, and eggs are filling the ovaries of girl babies. The lungs aren't working fully, but they're formed and developing.

The babies may even respond to sounds — such as music or your voice — by moving. Your twins' brains are growing rapidly. Though they're sleeping most of the time with eyelids closed , they can open their eyelids, which now sport eyelashes. They may even turn toward a light. Layers of fat are building up under their skin, smoothing it out.

They can make grasping motions and suck their fingers, and you may feel them hiccupping. Their lungs are continuing to develop. The twins are growing quickly and continue to gain fat. All of their bones are formed but still soft. They're practicing breathing rhythmically, moving the amniotic fluid in and out of their lungs to help them develop.

They can open and close their eyes, and nails now cover your babies' fingers and toes. Their arms, legs, and torsos are filling out. Some babies even have a full head of hair by now. At the same time, they're shedding the fine hair covering their bodies.

Control of their body temperature is one of the very last things to develop, but they're getting better at it, thanks to their brain development. While their skulls will remain soft allowing for an easier delivery , their other bones are hardening.

The average gestational age for twins at birth is 36 weeks. Their lungs are well developed by 34 weeks, and all body systems are now functioning. They spend their last few weeks building fat layers and putting on weight and shedding most of their body hair.

The vernix that waxy coating on their skin thickens, and body fat increases. BabyCenter's editorial team is committed to providing the most helpful and trustworthy pregnancy and parenting information in the world. When creating and updating content, we rely on credible sources: respected health organizations, professional groups of doctors and other experts, and published studies in peer-reviewed journals. We believe you should always know the source of the information you're seeing.

Learn more about our editorial and medical review policies. How your fetus grows during pregnancy. American College of Obstetricians and Gynecologists. Multiple pregnancy. Cleveland Clinic. Number of previous pregnancies — the greater the number of pregnancies a woman has already had, the higher her odds of conceiving twins. Heredity — a woman is more likely to conceive fraternal twins if she is a fraternal twin, has already had fraternal twins, or has siblings who are fraternal twins.

Race — Black African women have the highest incidence of twins, while Asian women have the lowest. Assisted reproductive techniques — many procedures rely on stimulating the ovaries with fertility drugs to produce eggs and, often, several eggs are released per ovulation. Fertilisation Hormones secreted by the ovaries, and a small gland in the brain called the pituitary, control the menstrual cycle. The average cycle is around 28 days. After a menstrual period, rising levels of the hormone oestrogen help to thicken the lining of the womb the endometrium and release an egg from one of the ovaries ovulation.

If the egg is fertilised on its journey down the fallopian tube, it lodges in the thickened womb lining, starts dividing and evolves into an embryo. This occurs because the fertilised egg divides in two while it is still a tiny collection of cells.

The self-contained halves then develop into two babies, with exactly the same genetic information. The biological mechanisms that prompt the single fertilised egg to split in two remain a mystery.

Approximately one quarter of identical twins are mirror images of each other, which means the right side of one child matches the left side of their twin. These babies will be no more alike than siblings born at separate times.

The babies can be either the same sex or different sexes, with the odds roughly equal for each. It is proposed that the egg splits in two, and each half is then fertilised by a different sperm. This theory is an attempt to explain why some fraternal twins look identical. Gestation for twins The normal length of gestation for a single baby is around 40 weeks.

However, gestation for twins, either identical or fraternal, is usually around 38 weeks. Since twins are usually premature, they are more likely to have lower birth weights. Prematurity is associated with increased risk of a number of disorders, including jaundice.

Giving birth to twins Childbirth can give rise to complications when just one baby is present, so twins present extra potential for difficulties. It is advised that women carrying twins give birth in hospital, rather than at home. The technical name for this is monoamniotic twins. For example, a twin who receives less blood from a shared placenta might weigh less at birth. Sharing a placenta means that twins share a blood supply during pregnancy. Sometimes the blood supply is shared unequally, which can cause health problems for both twins.

Women who are pregnant with twins sharing a placenta need to be checked more often than women with twins with separate placentas. Frequent checks can pick up early on any potential complications. Twins sharing an inner sac monoamniotic are also at a higher risk of complications during pregnancy because of the chance that their umbilical cords might tangle and cut off their blood supply.

These twins are checked even more closely. Medical professionals often recommend that these twins are born earlier than other types of twins. Medical professionals use ultrasound to work out how many placentas twins have.

The earlier the ultrasound, the more accurately it can say how many placentas there are. It gets harder to work out later in pregnancy. After the birth, the placentas should be looked at to confirm or determine what type of twins they are.

Same-sex twins with separate placentas can be fraternal or identical. To find out whether twins are identical or fraternal, you can ask for a genetic test after your babies are born. This is called a zygosity test.

Identical twins are more likely than fraternal twins to get the same illness. If one of a pair of identical twins is diagnosed with a particular disease or health condition, like high blood pressure, the other twin should be checked often for early symptoms.