Your baby’s growth: from conception to birth

Written by    Last updated: September 12, 2007

Find out about the long and complex period of growth, which turns a primitive ball of cells into a fully developed human. The embedded embryo develops a protective layer around him/her. This forms the basis of the placenta, the ‘organ’ that produces hormones to support the embryo and mother during pregnancy…

conception to birth

Find out how the miracle of life unfolds right from before conception until birth…

Before conception

Ovaries and the Ovarian cycle: Women are born with their entire stocks of eggs, or ova, in place. There are approximately 2 million ova present at birth, and they are either released or degenerate over the woman’s fertile life. The ovarian cycle is a woman’s reproductive cycle, lasting about 28 days, or one lunar month. At about 14 days before the end of the cycle, an ovum (or egg) is released from one of the ovaries. Women have two egg‐releasing organs, called ovaries, which release eggs alternately every month. This egg travels down the fallopian tubes into the uterus and begins to grow, releasing progesterone into its environment. It is at this stage that it is fertilisable, although it is not clear for how long. If the egg is not fertilised, it is shed with the lining of the uterus as the period begins.


Fertilisation: This occurs when a sperm fuses with the ovum. It takes place in the outer part of the Fallopian tube, generally within one day of ovulation. Only one sperm, out of the millions that are expelled, will actually penetrate the membrane covering the ovum. Once this occurs the surface of the zygote (the fertilised egg) changes, making it impossible to be penetrated by any other sperm.
Cell division: Within just a few hours of fertilisation the zygote will begin dividing itself and by the time it reaches the uterus, four days later, it has become a solid mass of hundreds of cells, called a morula. As time goes on, these cells continue to divide into a hollow mass of cells. At this time the cells are called a blastocyst.
Implantation: Around six days after ovulation, the blastocyst will release a hormone to help it bury itself into the lining of the uterus. The outer cells will begin joining with the small blood vessels in the uterus to form the placenta (afterbirth). Meanwhile, the inner cells begin to develop to form the embryo and the adjoining membranes. Once the blastocyst is completely imbedded, it begins to develop into an embryo. This is the state that the baby will stay in until he or she is born. Once the embryo is fully embedded, another set or hormones are released to trigger a number of changes. These changes occur to make the woman’s body suitable for carrying the embryo safely.
Multiple pregnancies: There are two ways that multiple pregnancies can come about. The first is that more than one egg is produced at one time. These eggs, if fertilised by different sperm, have their own placenta and develop side by side within the uterus. These babies will not be identical, as they have been formed from different eggs. The second way that a multiple birth can occur is when a single, egg, fertilised by one sperm, divides and develops into two embryos. These babies share the same placenta, and will be identical at birth. Triplets or larger numbers of multiple embryos are formed in one of these two ways, and will either be identical or not, depending on the form of conception.
Gender: The X and Y chromosomes, just two of the 46 that make up a baby, determine its gender. The eggs that are released every month carry only X chromosomes, while sperm carry both X and Y chromosomes. An egg fertilised by an X chromosome carrying sperm will become a girl (XX) baby, while an egg fertilised by a Y chromosome will become a baby boy (XY).

Baby’s growth month by month

The First Month
This is the beginning of a long and complex period of growth, turning a primitive ball of cells into a fully developed human. The embedded embryo develops a protective layer around him/her. This forms the basis of the placenta, the ‘organ’ that produces hormones to support the embryo during pregnancy, along with the rest of the embryo’s life support system. This is made up of three separate sections. The first is the amniotic sac, which is a bag full of fluid in which the embryo will float; the second is the chorion, which is a protective layer around the amniotic sac, and the third is the yolk sac, which will support parts of the baby’s development until its organs are able to function properly. These layers are vital for a baby’s growth, and support him/her through the in utero stages of development. The cells that make up the embryo begin to prepare themselves to form specific parts of the body, both internal and external.

Appearance: The being that was a cluster of cells has now become prawn‐shaped, and is about 4 millimetres long, and weighs less than a gram. It has a tiny, beating heart.

The Second Month

The cells that make up the embryo are moving to their correct places and rapidly multiplying to form a more recognisable baby‐shaped being. Developments are beginning to take place, hair and skin is beginning to grow, and the feet and legs are beginning to grow, with the arms and hands growing at a faster rate. The head is growing to contain the brain, which is developing at the fastest rate of all. This means that the head looks out of proportion to the rest of the body. The internal organs are beginning to form, and the heart, still tiny, beats very fast, at up to twice the rate of the mother’s. The embryo can now respond to touch.

Appearance: The embryo is now 2.5 centimetres long, and weighs 3 grams. The head is larger than the rest of the body, and looks a little out of proportion.

The Third Month

The baby is now fully formed in the womb, and is now called a foetus. The head is still large in proportion to the rest of his/her body, and it holds a fully formed face with complete eyes that are not yet open. The details of the body are developing; the finger and toe nails are in place and the genitals are beginning to grow, meaning that the gender of the baby is determinable from an ultrasound scan. Reflexes are also beginning to develop; the sucking, breathing and swallowing mechanisms are being rehearsed for life outside the womb. The umbilical cord is now fully mature, and the flow of nutrients and oxygenated blood from mother to baby, and the flow of waste products from baby to mother are now fully functioning. The baby is also able to make quite strong movements, although the mother will not feel these until a little later in the pregnancy.

Appearance: He/she is growing quickly. It is now 9 centimetres long and weighs approximately 48 grams.

The Fourth Month

The foetus is now starting to take on a more human form. His/her legs are now longer and in proportion with the rest of the body, and the head is also coming into line. The brain is capable of passing and receiving messages, but does not yet control the movements made by the now fully formed limbs. Some finer movements are also possible; his/her hands are now capable of grasping, and facial expressions are beginning to occur. The eyes are beginning to take on more detail, the eyebrows and lashes are growing, and the insides of the eyes are sensitive to light.

Appearance: The foetus is 13.5 centimetres long, and weighs 180 grams. The head still appears a little larger than the rest of the body.

The Fifth Month

There are some more detailed, internal changes taking place at this stage. The spinal cord has a protective layer around it, which is in place to prevent damage. The baby also has a primitive immune system in place that can defend against some infections. His/her skin is developing fast, and has a waxy secretion all over it to protect it from the fluid in the amniotic sac. The skin also has fine hairs all over it, called lanugo. The other main development at this stage is the addition of more functioning senses; the baby can now distinguish between bitter and sweet tastes, and can detect things touching his/her skin. His/her hearing is also more defined. The sounds of the uterus are becoming familiar, and he/she will respond to noises from outside the womb, and will often be especially receptive to rhythm and melody.

Appearance: There is not a great deal of difference in the size of the baby in this month; he/she is about 18.5 centimetres long and weighs about half a kilogram.

The Sixth Month

By this stage, the body is in proportion with itself, the head being of a regular size. His/her limbs are fully developed with the muscles and bones ready for the world outside. The sense of hearing is becoming sensitive to high frequency sounds, and his/her body is able to react to sounds around it. The lungs are developing in more and more detail, and breathing is practiced so the reflex is well rehearsed for the time that air is passed through them. By this point in the pregnancy, his/her brain is as developed as it will be at birth, and a sleep/wake routine is developed for the time in the womb.Appearance: Growth has slowed again; the foetus is about 25 centimetres in length, but his/her weight has doubled to 1 kilogram.

The Seventh Month

The baby’s brain grows larger in this month, and the cells and fibres become fully functioning. These changes in the brain spark preparations for birth. Fat grows under the skin to prepare for the colder temperature to come, his/her eyes open and the fine hair over the body recedes. The swallowing and sucking reflexes already in place are further rehearsed in preparation for life outside the womb, as is the breathing. He/she is also now able to regulate his/her body temperature. The other main change that takes place in this month is that the baby learns to orientate him/herself in the womb. This is in preparation for the time when it needs to position him/herself in the right place for labour to begin.

Appearance: The baby is now 28 centimetres long, and weighs approximately 1.5 kilograms.

The Eighth Month

Strong movements are now possible from fully coordinated limbs, but these movements are becoming more constricted, as there is less space in the womb. The mother will be able to feel these movements clearly. All organs except the lungs are fully developed. His/her eyes are reacting to the light coming from the walls of the uterus, and he/she is able to blink and focus. The fingernails are the full length, and there may be hair on his/her head.

Appearance: The baby is now about 32 centimetres long, about the same size as at birth, and he/she weighs about 2.5 kilograms.

The Ninth Month

It is in this month that the finer details are prepared for birth. The skin becomes smoother, and the eyes are now fully functional. All babies are born with blue eyes, but the colour may change a few weeks after birth. The reproductive organs are now fully formed, and virtually in position. A great deal of the debris from gestation ends up in the intestine. It is called meconium, and is a darkly coloured substance that is passed by the baby either during or after birth. The placenta is still fully active at this time, and the hormones being produced are still causing reactions in mother and baby. One hormone causes not only the mother’s breasts to produce milk, but also causes a swelling of the breasts of the baby, whether male or female. Once this hormone stops affecting a female baby, she may bleed lightly, in a similar way to a period, a few days after birth. Usually at this time the head ‘engages’ getting into place in the pelvis to prepare for birth. At the end of this period the baby is ready for birth. He/she should have everything that is required for life outside the womb, and should be able to breathe independently.

Appearance: The length of the baby has not changed a great deal; he/she will be about 35‐37 centimetres long, and will weigh anything from 3 kilograms upwards.

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