Twin-conception can occur for various reasons. In the first few weeks, the expectant mother usually has no physical clues to let her know that she has not one but two babies growing in her womb. Later on, she may go through heightened pregnancy symptoms (nausea, fatigue, breast-tenderness, appetite-swings); however, these symptoms don’t necessarily mean twins. The most reliable way to confirm twin-pregnancy, is through an ultrasound test around the sixth. Between weeks 4 - 8, the both the embryos grow to size of an inch or so and each weighs less than an ounce.
Their limbs, spines, livers, kidneys, bladders, brains and reproductive organs begin to develop at this time. Between 8 - 12 weeks, the development of twin embryos progresses swiftly, forming miniscule toes, fingers, eyelids and genitalia. Their size and weight increases by up to four times.
Till this time, twin pregnancies are not very different from singleton pregnancies.
Between 12 - 16 weeks, they measure around 6 inches and their kidneys start functioning. In the 16th week, the urine in their bladders and water in their stomachs, can be detected through the ultrasound. They begin to swallow the amniotic fluid and then pass urine. While they do move even at this stage, their movements are not strong enough to be felt by the mother. Between 16 - 20 weeks, the babies measure about 8 inches each and appear to be fully formed. However, their brains still continue to develop. Their movements are more coordinated, which prompts experts to believe that the babies are aware of each other by now. By the 20th week, the mother should be able to feel the babies kick, and move. Between 20 - 24 weeks, each twin should measure about 12 inches and should weigh about a pound and a half. Their footprints and fingerprints form at around this time and they are covered in protective hair called lanugo.
Between 24 - 28 weeks, each twin should measure about 15 inches and should weigh about 3 pounds each. Their features become distinct and they start accumulating fat under their skin. By the next four weeks, they should measure around 16 inches and weigh 4 pounds each. Their wrinkled skin should have filled out, with fat. However, their bones are still very soft and flexible. The size of their bodies also starts catching up to the size of their heads. Between 32 - 36 weeks, each baby should measure around 18 inches and weigh around 5 pounds, therefore, it may be a bit of a tight squeeze for the babies at this point. 38 weeks is considered to be a full-term pregnancy for twins. Their lanugo disappears and their bodies continue to store fat. The twins will barely have place to move around and will apply a lot of pressure on the placenta. Twins born at this time, may weigh a little less than singletons.
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