This is the official start of pregnancy and the last menstrual period. The 40 weeks of pregnancy are calculated by the first day of the last menstrual period.
The sperm meets the egg in the fallopian tube and only one of the sperm fertilizes the egg. There's a possibility that the egg with split in two to produce identical twins.
The cells in the fertilized egg divide while it floats down into the uterus. The rapid mitotic cell division of the zygote is called cleavage.
When the fertilized egg reaches the uterus, it burrows into the lining of the uterus. This process is called implantation. Now, the fertilized egg is known as a blastocyst with about 64 cells.
The embryo is now the size of a pin head and by the end of the week it wil have more than doubled in size. The placenta is developing and the embryo starts reveiving oxygen and nutrients.
All major organs and systems are forming, including the heart. The heart will begin to beat on the 25th day of conception.
The embryo's arms and legs begin to grow from the buds at each side. The baby's eyes and inner ears also start to develop.
The arms and legs continue to develop while the hands and feet begin to form. The mother will begin to feel the baby moving at week 20.
The arms and legs have lengthened. Their fingers and toes are forming, but are webbed.
From this week until birth, the embryo is called a fetus. The fetus is about the size of a strawberry. During this week, the fetus's fingerprints will form and the mouth and nose join.
The brain is growing and producing about 250,000 nerve cells every minute. The heart and the lungs are almost completely developed, but the fetus's blood is still oxygenated through the placenta.
The baby's brain and heart are fully formed, and the baby can also feel pain. Fingernails and toenails appear.
By now, the fetus is now producing insulin which controls their blood glucose levels. The fetus starts to flex its arms and kick its legs.
With the effect of hormones, the boy's prostate gland develops. For girls, their ovaries move from the abdomen into the pelvis.
The bone and marrow that make up the fetus's skeletal system are continuing to develop in this week. The bones and muscles are continuing to grow too.
The fetus has learned to breathe. It starts to inhale and exhale small amounts of amniotic fluid which helps the lungs to develop and grow. It also has more developed facial muscles.
Fat stores begin to develop under the fetus's skin this week. The fat will provide energy and help with temperature regulation after birth.
The fetus can hear its mother's heart beating, her stomach rumbling or blood moving through the umbilical cord. It makes the motions for cryinng, but no sound comes out.
If the fetus is female the uterus starts to develop, the vagina, uterus, and fallopian tubes are in place. If it's male, the genitals are recognizable in the ultra sound.
The fetus can now react to loud sounds and the mother will be able to feel the fetus moving. This week is important for sensory development because nerve cells serving the five senses are now developing in the brain.
Week Twenty One
The fetus's bone marrow starts to produce red blood cells. In females, the vagina is formed and the ovaries already contain over six million eggs. In males, the testes begin to descend from the abdomen.
Week Twenty Two
This week, the fetus's tastebuds begin to form on the tounge. The eyebrows and eyelids are fully developed.
Week Twenty Three
This week, the fetus gains a sense of balance. Its skin pigment is now forming and the pancreas is continuing to develop.
Week Twenty Four
The fetus's lungs are developing branches of the respiratory tree and cells that produce surfactant.
(The image is of a newborn receiving a surfactant treatment)
Week Twenty Five
The fetus's hands are now fully developed and it expores the uterus. If there are twins, then they explore each other.
Week Twenty Six
This is about the time when the fetus becomes most active. The fetus's hands are active and muscle coordination is such that it can suck its thumbs.
Week Twenty Seven
Now, and the fetus can open its eyes and it will react to the light At 27 weeks, the fetus's length will have tripled or quadrupled from the 14 week mark.
Week Twenty Eight
The branches of lungs are nearly complete. The fetus may be able to recognize its mother's voice.
Week Twenty Nine
This time the fetus's movements are becoming more forceful. Its muscles and lungs are continuing to mature as the fetus uses them. Its head has gotten bigger because so has the brain.
The fetus has gained more weight for temperature control. It now weighs about 3 pounds.
Week Thirty One
The fetus's lungs and digestive system are almost completed. Its eyes can now completely open.
Week Thirty Two
During this week, the reproductive development continues. In males, their testicles are moving into the scrotum. In females, their clitoris is now relatively prominent.
Week Thirty Three
The fetus is now growing hair and the lanugo disappears.
Week Thirty Four
The fetus continues to grow rapidly. The fetus is now around 18 inches long and weighs about 5.25 pounds.
Week Thirty Five
The fetus's reflexes are coordinated, and their five senses are more developed.
Week Thirty Six
The fetus is almost ready to be born. This week, the fetus may drop into the birth canal.
Week Thirty Seven
Usually, at this time, the fetus now measures 19.5 inches and weighs up to 7 pounds. The water could break at any time.
Week Thirty Eight
The fetus's development is now complete. The fetus may already be in its final birth postition. Now, the fetus needs to continue to gain weight.
Week Thirty Nine
The fetus has reached its final birth position.
The fetus is getting ready for birth and is settling into the fetal position.