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Pregnancy Stages and Gestational Milestones

Developing another life within the womb is an amazing feeling altogether – starting right from the moment of the formation of a little cluster of cells to a healthy bouncing baby, the entire process of pregnancy and development of the baby undergoes a range of stages. Pregnancy starts from the moment of fertilization and ends in the delivery of the baby, on an average 37 to 40 weeks later. Throughout this pregnancy period, the fetus slowly develops from an inconspicuous cluster of cells, gradually into a fully developed human baby.

Pregnancy Stages or Germinal Period of Pregnancy

Stage-1: Fertilisation in Pregnancy

The very first stage of pregnancy, rather conceiving the baby, is fertilization of the sperm and the egg and this is the first step in conception. The female body releases an egg by breaking the ovary follicle and sweeps down to the fallopian tube. The sperm of the male partner has to swim right from the vagina, up through the cervix and uterus and fertilise the egg in the fallopian tube.Amongst the 250 million sperms that a single ejaculation holds, only one sperm successfully manages to fuse with the egg.

Soon after the single successful sperm fuses with the egg, they fertilize into a new cell that starts to divide rapidly.

Stage-2: Development of Blastocyst & Implantation

In this stage of pregnancy, as soon as the fusion or fertilisation has taken place, the formation of the blastocyst starts occurring. The fused sperm and egg starts to collect the amniotic fluid from the body and envelop the fused cells. As this amniotic sac rapidly multiplies into more cells, it stars to look like a ball of cells, known as the blastocyst.

This formation consumes 5 to 8 days, after fertilisation. Within 9 to 10 days, this blastocyst is attached to the wall of the uterus. This process is called implantation in pregnancy.

Stage-3: Formation of Embryo & Placenta:

During implantation stage of pregnancy, the blastocyst forms one cell to four cells thick wall. The inner cells of this wall develop into an embryo. The outer cells on the other hand, burrow with the uterine wall and form the placenta. The placenta plays a very important role throughout the pregnancy and at this stage, it is this placenta that releases Human Chorionic Gonadotropin or HCG hormone, that determines the positivity in a pregnancy test. This hormone will prevent more eggs from being released and also continuously produce estrogen and progesterone hormones. It also carries water and minerals to the foetus and wastes from the fetus to the mother.

Stage-4: Formation of the Baby

In a typical pregnancy, the baby takes 37 to 40 weeks to develop and be delivered, from the first day of your last menstrual period. This entire process of development is divided into three stages and they are first trimester, second trimester, and third trimester. During each trimester or the three months period, or to specify, in every week, a special development occurs in the fetus and it leads the fetus to grow into a complete human baby.

Gestational Milestones or Embryonic Period in Pregnancy

Month ONE Gestational Milestones:

During the first three weeks of pregnancy, the process of fertilisation and the process of blastocyst formation occur. It is within the third week that the blastocyst and embryo are formed and implanted on the uterine wall. Even the placenta that is formed starts to provide the nutrition to the embryo and also take care of its health. From the fourth week, the basic growth or basic structures of the fetus start to develop. This includes the formation of head, chest, abdomen, brain, spinal cord, heart, internal organs and gastrointestinal tract. By the end of the fourth week the tiny “heart” tube of the baby will beat, 65 times in a minute. By this time, the baby should be ¼ inch long – that is smaller even than a grain of rice. This is an important gestational milestone.

Month TWO Gestational Milestones: Week 5 to Week 8

During this time, the gestational milestones seen are:

  • Umbilical cord is clearly visible.
  • Blood, kidney and nerve cells develop.
  • Arm and leg buds start to grow – they are visible, but not clearly distinguishable.
  • By the end of week 8, the lungs of the baby start to form.
  • Sex organs also begin to form.
  • The eyes move forward and the eyelids are also formed.
  • Looks more like a human baby and weighs less than 1/8th ounce and is 1 inch by this time.

Month THREE Gestational Milestones: Week 9 to Week 12

  • Muscles and nerves start to work together and the baby can now make a fist.
  • External sex organs can be visible through an ultrasound and can be defined by the test.
  • The previously formed eyelids now close to protect the eyes and they open only after the 28th week and the outer ears also take shape.
  • The intestines start to rotate and liver starts to make red blood cells.
  • The head is huge, double the size of the body of the fetus.

Fetal Development Period–Month FOUR Gestational Milestones: Week 13 to Week 16

  • The baby is no longer an embryo. It is now a fetus.
  • A more complete skeleton of the baby is formed in this pregnancy stage.
  • Skin of the fetus develops, but it is almost transparent.
  • In the intestinal tract of the baby, the first bowel or meconium is formed and liver and pancreas starts to secrete.
  • Baby starts to make movement and tries to stretch.
  • Lanugo or fine hair is seen to develop on the baby’s head.
  • Sucking motions and gestures can be seen on the baby’s mouth and tooth buds also get formed.

Month FIVE Gestational Milestones: Week 17 to Week 20

  • The baby can hear by this time.
  • The baby continues to float and rotate and becomes more active.
  • The movement of the baby or the fluttering feeling can be felt by the mother.
  • The baby can suck the thumbs.
  • The fetus is 6 inches long by this time and a baby bump can be seen at the lower abdomen of the mother.
  • The baby learns to swallow and yawn.

Month SIX Gestational Milestones: Week 21 to Week 24

  • Blood cells start to be made from the bone marrow of the baby.
  • Fingerprints and footprints of the baby are developed and are clearly visible.
  • Taste buds are formed on the tongue of the baby.
  • Lung formation is complete, but they do not work.
  • Baby sleeps and wakes frequently and feels reflex.
  • The internal reproductive organs are developed – for baby boys, scrotum is formed and for baby girls, ovaries are developed with a lifetime supply of eggs.
  • The baby is 12 inches long by this time and is 1½ pound in weight.

Month SEVEN Gestational Milestones: Week 25 to Week 28

By this time, the baby is 2½ pound in weight and it can sense its position inside the mother’s womb. The baby might have hiccups sometimes that the mother will be able to feel. It is at this gestational stage that the mother requires to undergo a Doppler study and ultrasound, as the third trimester starts.

Month EIGHT Gestational Milestones: Week 29 to Week 32

  • The baby is completely developed in terms of bone structure, but the bones are still soft.
  • Baby can kick and jab.
  • The baby’s eyelids open and close with change of light.
  • Baby’s body begins to store vital minerals like iron and calcium.
  • Baby weighs about 4 to 4½ pounds.
  • Lungs of the baby start to practice breathing movements.
  • Lanugo or fine hair on the baby starts to fall off.
  • Baby is about 15 to 17 inches long.

Month NINE Gestational Milestones: Week 33 to Week 36

  • The body of the baby is coated with a thick waxy coat called vernix.
  • At this pregnancy stage, the baby is about 16 to 19 inches long.
  • The body fat increases of the baby.
  • The baby weighs about 6 to 6½ pounds.

Month TEN Gestational Milestones: Week 37 to Week 40

  • By this pregnancy stage, the baby is fully developed and is ready to be delivered at any time
  • The baby will be 19 to 21 inches in length.
  • The baby can weigh somewhere between 6 ¾ lbs to 10 pounds.

Depending on the condition or the growth of the baby, within the weeks of 38 to 42, a fetus is ready for vaginal delivery. In cases, a vaginal delivery is not possible; the doctor does a C section or Caesarean section. If the baby is born before 37 weeks of pregnancy then it is known as premature delivery. If the baby is born after 40 weeks of pregnancy then it is considered late term pregnancy.

Changes in the Mother During Different Pregnancy Stages:

During the three trimesters or pregnancy stages, the mother goes through many changes. Right from the moment of early signs of pregnancy, to the delivery of the baby, the mother will experience various changes in her mental and physical condition.

In the FIRST TRIMESTER or Week 1 to Week 12, the mother will experience:

  • Miss the period, feel tender and swollen breasts and extreme tiredness.
  • Stomach upset, morning sickness and constipation.
  • Headache, heartburn and craving or distaste for certain foods will occur.
  • Frequent need to pass urine.
  • These symptoms will go away as the pregnancy progresses and many mothers might not feel any pregnancy symptoms at all.

In the SECOND TRIMESTER or Week 13 to Week 28, the mother will experience:

  • During this pregnancy stage, the mother feels thigh pain, abdominal pain and pain in the back and groins.
  • Stretch marks will develop in the lower abdomen, breasts and buttocks as well as thighs.
  • The mother’s nipples will grow darker in skin colour.
  • During this pregnancy stage, the mother experiences carpal tunnel syndrome or numbness in hands.
  • During this pregnancy stage, the mother feels itching on feet, palm and abdomen.
  • During this pregnancy stage, the mother has swelling on face, fingers and ankles.

In the THIRD TRIMESTER or Week 29 to Week 40, the mother will experience the second trimester symptoms along with:

  • Sleeping and breathing troubles can be experienced by the mother in this stage of pregnancy.
  • There is frequent need to urinate, as the baby grows bigger and puts pressure on your internal organs in this stage of pregnancy.
  • The mother feels the baby ‘dropping’ or moving towards the lower abdomen.
  • The mother’s breasts start to leak pre-milk or colostrums.
  • The mother’s belly button will stick out.
  • The mother will start experiencing contractions, indicating real as well as false labour pain.

You are about to reach the due date of delivering your baby. In case of vaginal or normal delivery, you will feel the cervix becoming thinner to help the baby to come down. This is normal and you will be assisted by the doctors. It is time to enjoy the blissful outcome of pregnancy!


  1. American Pregnancy Association: https://americanpregnancy.org/
  2. Mayo Clinic: https://www.mayoclinic.org/
  3. WebMD: https://www.webmd.com/
  4. Centers for Disease Control and Prevention (CDC): https://www.cdc.gov/
  5. National Institute of Child Health and Human Development (NICHD): https://www.nichd.nih.gov/

Also Read:

Pramod Kerkar, M.D., FFARCSI, DA
Pramod Kerkar, M.D., FFARCSI, DA
Written, Edited or Reviewed By: Pramod Kerkar, M.D., FFARCSI, DA Pain Assist Inc. This article does not provide medical advice. See disclaimer
Last Modified On:August 2, 2023

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