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The first trimester: Your baby's growth and development in early pregnancy

A pregnancy has plenty of milestones, early signs of pregnancy, week-by-week, month-by-month and three month blocks called trimesters. Here's what's happening during the first trimester.

Month one of pregnancy

The number of weeks of the length of a pregnancy start from the first day of the last period, so for part of the first month of pregnancy, a woman isn't actually pregnant.

As the fertilised egg grows, a water-tight sac forms around it, gradually filling with fluid. This is called the amniotic sac, and it helps cushion the growing embryo throughout pregnancy.

The placenta also develops at this point in the first trimester. The placenta is a round, flat organ that transfers nutrients from the mother to the baby, and transfers waste from the baby.

A primitive face takes form with large dark circles for eyes. The mouth, lower jaw, and throat are developing. Blood cells are taking shape, and circulation will begin.

By the end of the first month of pregnancy, your baby is around 6-7mm (1/4 inch) long - about the size of a grain of rice!

end of first month

Month two of pregnancy

Your baby's facial features continue to develop. Each ear begins as a little fold of skin at the side of the head. Tiny buds that eventually grow into arms and legs are forming. Fingers, toes, and eyes are also forming in the second month of pregnancy.

The neural tube ( brain, spinal cord, and other neural tissue of the central nervous system) is well formed. The digestive tract and sensory organs begin to develop. Bone starts to replace cartilage. The embryo begins movement although the mother cannot yet feel it.

By the end of the second month, your baby, now a foetus, is about 2.54cm (1 inch) long, weighs about 2g and a third of baby is now made up of its head.

6 Weeks

At about 6 weeks, your baby's heartbeat can usually be detected.

Month three of pregnancy

Your baby has arms, hands, fingers, feet, and toes and can open and close its fists and mouth. Fingernails and toenails are beginning to develop and the external ears are formed. The beginnings of teeth are forming. Your baby's reproductive organs also develop, but the baby's gender is difficult to distinguish on ultrasound. The circulatory and urinary systems are working and the liver produces bile.

At the end of the third month, your baby is about 6cm long and weighs between 9-14g.

12 Weeks

Since your baby's most critical development has taken place, your chance of miscarriage drops considerably after three months into the pregnancy.

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WebMD Medical Reference

Medically Reviewed by Dr Rob Hicks on June 21, 2017

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