Your Baby
Week by Week
A Pregnancy Guide
Your Baby: Weeks 1-8
Your baby is growing and changing every week after conception:
This is the most critical period of the pregnancy.
During the first week, the development of your baby starts immediately after fertilization (when the male
sperm and female oocyte (egg) meet in and unite to form the zygote). If more than one egg is released from
the ovary, twins may develop. The developing zygote has 46 chromosomes – 23 from you and 23 from the
father. These chromosomes will ultimately dictate the baby’s gender, size, hair and eye color, etc.
From there things happen quickly. These two cells start to multiply and will migrate from the tube where they
met to the uterus where they will implant around week 4. The placenta then starts to develop which will
supply all of the nourishment the developing fetus will need.
Weeks three through eight are called the Emrbyonic Period. Most, if not all, of the major tissues and organs
are formed during this time. During this time the neural tube (or spinal cord) will close. Taking folic acid
supplements through prenatal vitamins will help prevent neural tube defects like Spina Bifida at this point in
your pregnancy.
By the end of this period, an ultrasound can be used to measure the crown-rump length – from head to tail.
And at 7-8 weeks, the embryo will measure 26-30 mm, the size of a large almond. The limbs have started to
develop and the heart is beating at around the 6th week.
At the completion of the Embryonic Period - the neural tube, the heart, and all major organ systems have
formed. This entire process is called organogenesis.
Your Baby: Weeks 9-12
Your baby is growing and changing every week after conception:
Now that the Embryonic Period is complete – week 9 kicks-off a very rapid growth phase and the Embryo is
now referred to as a Fetus.
During this 4-week period – the limbs continue to grow as the bones develop; the head becomes more round
and is half the size of the entire body length. At week 9, the genitals start to differentiate into male or female
and which will be completed by week 12.
By the end of the Fetal Period, the fetus will measure 50-70 mm (2-3 inches) or the size of an apricot.
Your Baby: Weeks 12-16
Your baby is growing and changing every week after conception:
The beginning of the second trimester brings with rapid growth – this continues for the entire fetus except for
the fetal head.
The face becomes more defined with the eyes now lie directly on the front of the face, the ears have moved to
their final location on the side of the head.
The bones are formed completely and the skeleton can be seen well for the first time on x-ray.
The fetus may start sucking its thumb and fetal movements will become coordinated. By the end of this four
weeks, the fetus will measure approximately 9-14 cm (4-6 inches) and will weigh about 200 grams (just less
than 1/2 pound) or the weight of a small potato.
Your Baby: Weeks 16-20
Your baby is growing and changing every week after conception:
The rapid growth slows down and the extremities become proportional to the rest of the size of the fetus.
Lanugo (fine hair) and vernix caseosa form to cover and protect the skin.
The spinal cord is now myelinated (the surface of many nerve fibers) and most importantly for you – you may
start to feel the baby move. This is called quickening.
The fetus will weigh 400-450 grams (1 pound) by the end of 20 weeks.
Your Baby: Weeks 20-24
Your baby is growing and changing every week after conception:
The head of the fetus is becoming more normal in size compared to the rest of the body.
The fingernails, eyelids and eyebrows are well developed and the Lanugo darkens.
The skin is more wrinkled due to not having much fat and the excess growth of the skin.
The face and body take on the appearance of an infant seen at birth. Usually by the end of the 24th week, the
fetus will weigh 500-700 grams (1-1/12 pounds) and is considered “viable” – or able to live outside of the
uterus should it be born at this point.
This is not always the case, as the coordination between the central nervous and respiratory systems is not well
organized.
Your Baby: Weeks 24-30
Your baby is growing and changing every week after conception:
During the beginning of the third trimester, the fetus continues to grow and the organ systems become better
coordinated.
The lungs are becoming more developed with the formation of the terminal sacs that will aid in oxygen
exchange. The eyes can open between 26-29 weeks, and the eyelashes are developed.
The hairs on the scalp are growing in and the baby is now becoming chubby with the addition of additional fat
deposits.
The baby at this point will weigh 900-1200 grams (approximately 2 lbs) and will be 9 ½ to 10 ½ inches long.
Your Baby: Weeks 30-34
Your baby is growing and changing every week after conception:
The body continues to add fat and the skin is now pink.
The toenails are present and the fingernails reach the end of the fingers.
The male fetus’ testes will start to descend into the scrotum. The baby will weigh 1400-1800 grams (2 ½
pounds) and will be 28-32 cm or approximately 1 foot long.
Your Baby: Weeks 34-37
Your baby is growing and changing every week after conception:
The plump baby is getting ready for delivery.
The toenails have reached the ends of the toes, the Lanugo has been completely shed, vernix continues to
protect the skin, the testicles have reached the scrotum, the ovaries have not yet reached the pelvis (and may
not before birth).
The brain begins its myelination process
Your Baby: Weeks 37-40
Your baby is growing and changing every week after conception:
The baby is finally full-term and will weigh on average 3,200-3,600 grams or 6-7 pounds.
Your Baby: Weeks >40
Your baby is growing and changing every week after conception:
Post-term delivery of the infant is common and your Obstetrician will discuss the risks of continuing the
pregnancy beyond 41-42 weeks due to concern for placental problems and large babies which may complicate
labor and child birth.