Pregnancy timeline. The countdown of human development begins at week 3
after the woman's menstrual period. When counted this way, pregnancy will last a total of 40 weeks.
Your baby's development. Follow along the timeline and learn about significant events in your baby's development.
Week 1: Pre-pregnancy. The weeks of pregnancy are counted from the start of the
woman's last menstrual period. When your menstrual period begins and the lining of the uterus is shed.
Hormones in your body are preparing another egg for release.
Week 2: Ovulation. Once a month, in response to hormones, one of the ovaries expels a mature egg.
This happens about mid-cycle. The lining of the uterus is becoming thick again in order to receive a fertilized egg.
Week 3: Fertilization. The egg cell is released into the fallopian tube and
fertilized by a sperm cell. The fertilized egg cell, called an embryo, divides into two cells, four cells,
and so on, forming a tight ball. It moves toward the uterus and implants into the uterine wall.
Week 4: Implantation. Approximately 6 to 10 days after fertilization, the embryo implants into the
lining of the uterus. Special networks of blood vessels form between the embryo and the walls
of the uterus. The embryo will then be able to receive nutrients from the mother's blood.
Month 2, Week 5: The yolk sac has formed which provides nutrients to the embryo
while the placenta develops. The embryo lengthens from a ball to a slipper shape.
The major organs begin to form including the neural tube, which will develop into the central nervous system.
Month 2, Week 6: The embryo's brain and spinal cord grow rapidly
causing the embryo to take a curled up position. The neural tube is now fully formed.
Little buds appear on the embryo's body, which will become the arms, legs, and other structures.
Month 2, Week 7: Brain development is rapid this week. The face, nose, ears, and eyes begin to develop.
The arm and leg buds continue to form and the ends of the upper arms become paddle shaped.
The embryo may start to twitch.
Month 2, Week 8: The back of the brain grows rapidly causing the head to become much larger than the body. Little buds form at the ends of the limbs which will become
fingers and toes. The ears develop close to the neck and the eyes develop on the sides of the head.
Month 3, Week 9: The upper and lower limbs are undergoing a lot of changes. Notches appear
between the buds at the ends of the hands, clearly marking the beginnings of fingers.
Month 3, Week 10: By the end of week 10 the embryo looks distinctly human,
but its head is still very large compared with the body. The neck is visible and the eyelids are formed.
The outer ears almost look like real ears. They are set quite low on the sides of the head.
Month 3, Week 11: At week 11, the face is broad and the eyes are widely separated on the face.
The eyelids are fused and the ears are still low set close to the neck.
The major organs are still developing.
Month 3, Week 12: Around this time, the fetus starts to make urine and begins to transfer waste products to the mother through the placenta.
Month 3, Week 13: Most of the major organ systems have developed to some extent.
The central nervous system, heart, limbs, eyes, and ears are in critical stages of development.
Month 4: Weeks 14 to 17: By week 14 the genitals are well developed. By week 16
your baby is completely formed. Limb movements become coordinated.
By week 17, your baby may move their eyes and show startle reflexes in response to loud noises.
Month 5, Weeks 18 to 21: By week 18 your baby starts to suck and swallow.
Their tooth buds have appeared and their fingers and toes are well defined.
They now get their nourishment from the placenta. Their skin is thin and there is little underlying fat.
Month 6, Weeks 22 to 26: Your baby gains a substantial amount of fat this month
which helps them retain body heat. Now their heartbeat can be heard by an ordinary stethoscope,
although several organ systems are now functioning, their lungs have not yet fully developed.
Month 7, weeks 27 to 30: Space starts to get tight in the womb and your baby takes on a curled up
position with their arms and legs crossed. Their central nervous system is mature and can direct
breathing and control body temperature.
Month 8, Weeks 31 to 35: Your baby gains half a pound, or 230 grams, every week now. They are probably head down in the uterus. Their movements will peak in week 32.
Their organs are almost completely developed, although their lungs may still be immature.
Month 9, Weeks 36 to 40: This month, your baby is gaining a lot of fat each day.
The baby's head will begin to settle downward deep inside the pelvis as it prepares for birth.