Fertilisation to adulthood – Timeline
This timeline gives information on the key stages in the development of a human.
Please note: The weeks in pregnancy are typically counted from the 1st day of a woman’s last menstrual cycle. This is referred to as gestational age. In this timeline, the dates listed for embryo and foetus are counted from fertilisation. This is referred to as embryonic age.
Human development, prenatal – Fertilisation to birth
The prenatal period is the time between fertilisation and birth. In humans, this is typically 38 weeks.
Fertilisation – Zygote
An egg and sperm fuse and the zygote moves down the mother’s fallopian tubes. It undergoes rapid cell division without increasing in size and becomes a blastocyst. It attaches to the wall of the uterus and becomes embedded in the lining (endometrium).
~1 week – Embryo
The embryo receives nutrients via the placenta and is protected by amniotic fluid. It divides into head and trunk. Brain, spinal column and internal organs begin to develop. At this stage, the embryo is very sensitive to environmental factors such as alcohol and drugs.
How does a cell decide what to be?
Cells decide what they will become in the embryo. Associate Professor Peter Dearden, from the University of Otago, is interested how our genes control this process.
~8 weeks – Foetus
The foetal stage lasts from 8 weeks post fertilisation until birth. Organ development continues throughout this period. A foetus is considered full term between 37 and 40 weeks, but some survive from as early as 22–24 weeks with advanced medical technology.
Ultrasound of an unborn baby
Ultrasound is a medical imaging technique that uses high frequency sound waves and their echoes. The sound waves are above human hearing, and the echoes from the sound waves bouncing off tissue inside the body are turned into electrical pulses that can be put together into a digital image, such as this one of an unborn baby.
Newborn – Neonate
A period of rapid change. Congenital abnormalities may appear. Nutrition and oxygen no longer come from the umbilical cord but the newborn is still dependent on an adult for nutrition. Able to focus eyesight over a short distance. Head very large in proportion to body.
1 month – Infant
An intense period of development. Key milestones for an infant include raising their head, grasping objects, rolling from stomach to back, sitting unsupported, crawling, making babbling noises, getting first teeth and eating solid foods.
1 year – Toddler
An intense period of development. Key milestones are learning to walk, self-feeding and beginning to use words to communicate.
3 years – Early childhood
An intense period of development. Key milestones include becoming toilet trained, development of both gross motor skills (for example, balancing on 1 foot) and fine motor skills (for example, holding a pencil). Social and behavioural skills develop.
5 years – Childhood
Growing taller and bigger. An important period for cognitive development. Social and behavioural skills develop.
10 years – Prepubescence
This period is often marked by accelerated growth. Puberty may begin towards the later stages.
12 years – Puberty
During this period, most females will begin to menstruate. Males may start to grow facial and body hair, and the pitch of their voice drops. In both sexes, pubic hair appears and genitals and reproductive organs mature.
20 years – Early adulthood
Brain development continues. Further secondary sexual characteristics develop in males – more pubic and facial hair, larger hands and feet, broader shoulders and chest, larger skull and bone structure, greater muscle mass, prominent Adam’s apple, deeper voice.
40 years – Middle adulthood
Signs of ageing in this period include loss of skin elasticity, greying hair, waning of physical fitness and decline in fertility. Menopause in women.
60 years – Advanced adulthood
Increased signs of ageing – wrinkles, liver spots on skin, loss of hair, diminished hearing and eyesight, slower agility, slower reaction times, memory loss, greater susceptibility to bone diseases. Average New Zealand life expectancy – 82.4 for females, 78.4 for males.
Grandfather and grandson
Older New Zealanders make important contributions to our society, including sharing their knowledge and time with younger generations. The Ageing Well National Science Challenge will improve the health of New Zealanders as they age so they continue to play a vital role in society.
