The countdown to baby begins this week. The only thing is there is no baby in sight or inside. Why then call this week 1 of pregnancy? The reason being it's extremely difficult to pinpoint the precise moment when the sperm meets the egg.
What isn't hard to pinpoint is the first day of your last menstrual period. And conception typically occurs about two weeks after your period begins.
To calculate your due date, you can count ahead 40 weeks from the start of your last period. This means you clock in two weeks of your 40-week pregnancy before you even get pregnant!
There is still no sign of baby, but your body is working hard to gear up for the big ovulation. Your uterus wall is thickening and your ovarian follicles are maturing, until one egg becomes the dominant one destined for ovulation.
Then it will make its journey down your fallopian tube in search of the one lucky sperm out of the millions to seal the deal.
You have conceived, congratulations! This means the sperm and egg has united in one of your fallopian tubes to form a one-celled entity called a zygote. Within hours, the zygote divides and within days, your baby-to-be will become a microscopic ball of cells. In the case where more than one egg is released and fertilised, you may have multiple zygotes.
The zygote has 46 chromosomes (23 from you and 23 from your partner) which will help determine your baby's sex, traits such as eye and hair colour, and, to some extent, personality and intelligence.
Soon after fertilisation, the zygote begins its journey down the fallopian tube toward the uterus. At the same time, it will begin dividing rapidly to form a cluster of cells where the inner group of cells will become the embryo. The outer group of cells will become the membranes that nourish and protect it.
It's time for implantation! The ball of cells has reached your uterus and is now called the embryo. Once firmly in place on the uterine lining, the ball of cells undergoes some division - half will become your baby while the other half will become the placenta, your baby's lifeline throughout the pregnancy.
By the end of this week, you may be celebrating a positive pregnancy test.
The embryonic period
The fifth week of pregnancy (or third week after conception) marks the beginning of the embryonic period. This is when the baby's brain, spinal cord, heart and other organs begin to form.
The embryo is now made of three layers. The inner layer (endoderm) will develop into your baby's digestive system, lungs and liver. The middle layer (mesoderm), will soon be your baby's heart, bones, kidneys, muscles and sex organs. The outer layer (ectoderm) will eventually form your baby's central and peripheral nervous systems, hair, skin, eyes and many connective tissues.
By the end of this week, your baby is probably between 1.5 to 3 millimetres - about the size of the tip of a pen.
The neural tube closes
Your baby is growing rapidly this week. Just four weeks after conception, the neural tube along your baby's back is closing and its heart is pumping blood.
This week also sees basic facial features beginning to appear - baby's jaws, cheeks and chin, including passageways that will make up the inner ear and arches that will contribute to the jaw. Your baby's body begins to take on a C-shaped curvature. Small buds will soon become arms and legs.
Your baby's tiny heart is beating about 80 times per minute and getting faster by the day.
By the end of this week, your baby may be 4 to 6 millimetres long from crown (head) to rump (bottom), no bigger than a nail head.
Baby's head develops
Seven weeks into your pregnancy (or five weeks after conception), your baby's mouth and tongue are forming. During this period of development, your baby's arm and leg buds into paddle-like appendages to divide into hand, arm and shoulder segments, and leg, knee and foot segments.
At this stage, your baby's brain and face are rapidly developing. Tiny nostrils become visible, and the eye lenses begin to form. Also in place are your baby's kidneys which will soon begin their important work of waste management.
By the end of this week your baby may be 7 to 9 millimetres long - about the size of a blueberry.
Eight weeks into your pregnancy (or six weeks after conception), your baby's arms and legs are growing longer, and fingers have begun to form.
Your baby is looking less reptilian and more human as your baby's ears begin to take shape, his/ her eyes and nipples become visible. The trunk of your baby's body is beginning to straighten.
Your baby's heart is beating at an incredible rate of 150 times per minute. Your baby may begin to move this week, but you won't be able to feel it yet.
By the end of this week, your baby may be about 11 to 14 millimetres long - about the size of a large raspberry.
Baby's toes form
In the ninth week of pregnancy (or seven weeks after conception), your baby's arms grow, develop bones and bend at the elbows. Toes begin to form, and your baby's eyelids and ears continue developing.
This week, tiny muscles are starting to form which will allow your baby to move his/ her arms and legs, though it may be another month before you are able to feel those little punches and kicks.
By the end of this week, your baby may be close to about 18 to 22 millimetres long - the size of a medium green olive.
Baby's neck begins to develop
By the tenth week of pregnancy (or eight weeks after conception), your baby is growing by leaps and bounds. Your baby's head has become more round. The neck begins to develop, and your baby's eyelids begin to close to protect his/ her developing eyes.
Bones and cartilage are also forming and small indentations on the legs are developing into knees and ankles. Tiny buds of your baby teeth are forming under the gums. The stomach and kidneys are functioning more. Although the genitals are developing, it's still too soon to identify your baby's sex.
By the end of this week, your baby may be about 35 millimetres long - the size of a prune.
Baby's genitals develop
At the beginning of the eleventh week of pregnancy (or the ninth week after conception), your baby's head still makes up about half of its length but its body is about to catch up. Your baby's body is straightening out and the torso is lengthening. Your baby is now officially described as a foetus.
This week your baby's eyes are widely separated, the eyelids fused and the ears low set. Red blood cells are beginning to form in your baby's liver. The nails are forming on individual fingers and toes, having separated recently from the webbed hands and feet.
This week, your baby's external genitalia will start developing into a penis or clitoris and labia majora.
By now, your baby may measure about 50 millimetres long from crown to rump and weigh almost 8 grams.
Baby's fingernails develop
Twelve weeks into your pregnancy (or 10 weeks after conception), your baby has more than doubled in size. The digestive system is beginning to practice contraction movements, the bone marrow is making white blood cells and the pituitary gland at the base of the brain has started producing hormones. Also, your baby's face now has a human profile.
By now, your baby may be nearly 61 millimetres long from crown to rump and weigh about 14 grams - the size of a large fresh plum.
Thirteen weeks into your pregnancy (or 11 weeks after conception), your first trimester comes to a close. This is the time when your baby's intestines have moved from the umbilical cord to your baby's abdomen. Your baby is also beginning to form urine and discharge it into the amniotic fluid.
Tissue that will become bone is also developing around your baby's head and within his/ her arms and legs. Tiny ribs may soon appear. Also developing this week is your baby's vocal cords.
By now your baby may be about 75 millimetres long - the size of a peach.
Baby's sex becomes apparent
Fourteen weeks into your pregnancy (or 12 weeks after conception), the start of your second trimester, your baby's arms have almost reached the length they will be at birth and your baby's neck has become more defined. Red blood cells are forming in your baby's spleen.
Eyebrow hair is also filling in, as is body hair called lanugo. And on top of your baby's little head, he/ she might be actually sprouting some hair. Your baby's sex will become apparent this week or in the coming weeks.
By now your baby may be almost 87 millimetres long from crown to rump and weigh about 45 grams - the size of your clenched fist.
Skeleton develops bones
Fifteen weeks into your pregnancy (or 13 weeks after conception), your baby is growing rapidly. Your baby's skeleton is developing bones, which will become visible on ultrasound images in a few weeks.
By now, your baby has coordination, strength and starts to wiggle his/ her fingers and toes and may suck a thumb, all in preparation for the big debut into life outside the womb.
Your baby is certainly getting a workout - knocking, flexing and moving his/ her arms and legs.
By now, your baby may be about 95 millimetres long - the size of a navel orange.
Possible facial expressions
Sixteen weeks into your pregnancy (or 14 weeks after conception), your baby's eyes have begun to face forward and slowly move. The ears are close to reaching their final position. More-developed facial muscles may lead to various expressions, such as squinting and frowning. Though the eyelids are sealed, your baby's eyes are making small side-to-side movements and can perceive some light.
Although still too slight to be felt, your baby's movements are becoming coordinated and can be seen during ultrasound exams.
By now your baby may be more than 120 millimetres long from crown to rump and weigh nearly 110 grams.
Seventeen weeks into your pregnancy (or 15 weeks after conception), body fat is beginning to form and store under your baby's skin. The fat will provide energy and help keep your baby warm after birth.
Your baby's heart rate is regulated by the brain and clocks in at 140 to 150 beats per minute.
By now your baby may be more than 130 millimetres long from crown to rump and weigh nearly 140 grams.
Baby begins to hear
Eighteen weeks into your pregnancy (or 16 weeks after conception), your baby's ears begin to stand out on the sides of his/ her head. As the nerve endings from your baby's brain "hook up" to the ears, your baby may hear your heart beating, your stomach rumbling or blood moving through the umbilical cord. He/ she may even be startled by loud noises.
Another set of skills your baby is mastering is yawning and hiccupping. Your baby will be one-of-a-kind as it is now complete with unique fingerprints on his/ her fingertips and toes.
By now your baby may be 140 millimetres long from crown to rump and weigh 200 grams.
Baby's uterus forms
Nineteen weeks into your pregnancy (or 17 weeks after conception), your baby's hearing continues to improve. He/ she may pick up your voice in conversations — although it's probably hard to hear clearly through the amniotic fluid and protective paste covering your baby's ears.
For girls, the uterus and vagina may be forming this week.
By now your baby may be more than 150 millimetres long from crown to rump and weigh about 240 grams.
Halfway into your pregnancy (or 18 weeks after conception), your baby's delicate skin is protected with a greasy, cheese-like coating called vernix caseosa. You may be able to feel your baby's first movements, also known as quickening.
If you want to know, you can go for an ultrasound scan to detect if your baby is a boy or a girl. If you are having a girl, her uterus is fully formed, her ovaries and vaginal canal are starting to develop. If you are having a boy, his testicles have begun their descent from the abdomen and will drop into the scrotum.
At this stage, your baby will be twisting, turning, kicking and punching and doing an occasional somersault.
By now your baby may be about 160 millimetres long from crown to rump and weigh about 280 grams.
Baby can swallow
Twenty-one weeks into your pregnancy (or 19 weeks after conception), your baby is becoming more active and can swallow. He/ She is getting a taste of whatever's on your menu.
Arms and legs are finally in proportion, neurons are now connected between the brain and muscles and cartilage throughout the body is turning to bone. This means your baby's movements are more coordinated, no more jerky twitches.
By now your baby may be about 170 millimetres long from crown to rump and about 320 grams.
Baby's hair becomes visible
Twenty-two weeks into your pregnancy (or 20 weeks after conception), and your baby has grown rapidly. He/ She is completely covered with a fine, down-like hair called lanugo. The lanugo helps hold the vernix caseosa on the skin. Your baby's eyebrows may be visible.
Your baby is also developing his/ her senses - touch, sight, hearing and taste. Your baby may grab the umbilical cord, perceive light and dark, listening to voices and tasting the food you are eating.
By now your baby may be 190 millimetres long from crown to rump and weigh about 460 grams (or 1 pound)
Fingerprints and footprints form
Twenty-three weeks into your pregnancy (or 21 weeks after conception), and your baby's skin is wrinkled. This is so as skin grows faster than fat develops and there is not much fat to fill the skin out yet. Your baby is also more translucent than before and pink to red in colour.
This week your baby begins to have rapid eye movements. Your baby's tongue will soon develop taste buds. Fingerprints and footprints are forming. For boys, the testes are beginning to descend from the abdomen. For girls, the uterus and ovaries are in place - complete with a lifetime supply of eggs.
By now your baby may be about 200 millimetres long from crown to rump and weigh more than 460 grams.
Real hair grows
Twenty-four weeks into your pregnancy (22 weeks after conception) and your baby is regularly sleeping and waking.
Your baby is putting on weight as well from growing organs, bones and muscles. His/ Her tiny face is almost fully formed and real hair is beginning to grow on your baby's head.
By now your baby may be slightly longer than 210 millimetres from crown to rump and weigh more than 630 grams.
With intensive medical care, some babies born this week may be able to survive.
Twenty-five weeks into your pregnancy (23 weeks after conception) and your baby's air sacs are now lined with tiny capillaries in the lungs for him/ her to take the first breath of fresh air. However, they are still underdeveloped to sufficiently send oxygen to the blood stream and release carbon dioxide from the blood.
Your baby's nostrils, which have been plugged up to now, are starting to open up this week. Also, your baby's vocal cords are functioning now, leading to occasional hiccups (which you may get to feel).
Also, your baby's hands are fully developed - although the nerve connections to the hands have a long way to go. Exploring the structures inside your uterus may become baby's prime entertainment.
By now your baby may be about 230 millimetres and weigh more than 670 grams.
Baby's eyes develop
Twenty-six weeks into your pregnancy (or 24 weeks after conception) and your baby's eyelids that have been fused for the past few months are beginning to open. Your baby is able to see, though not much in the dark confines of his/ her uterine home.
Your baby has a heightened sense of sight and hearing and a loud vibrating nose close to your belly may cause your baby to respond by blinking or jerking.
By now your baby may be more than 230 millimetres long and weighs nearly 820 grams.
Second trimester ends
This week marks the end of the second trimester. At 27 weeks (or 25 weeks after conception), your baby's lungs, liver and immune system are continuing to mature. From this week, your baby will not be measured from crown to rump but head-to-toe length.
Your baby has more taste buds now that he/ she was at birth. This means that not only is your baby able to taste the difference in the amniotic fluid, he/ she may even react to it by hiccupping or kicking.
By now your baby about 380 millimetres long and weighs just over 850 grams.
Baby's eyes open
Twenty-eight weeks into your pregnancy (26 weeks after conception), your baby's eyelids are partially open and eyelashes have formed. Your baby has also acquired the skill of blinking and other tricks, like coughing, sucking, hiccupping and taking practice breaths.
Your baby's lungs are nearly mature by now but he/ she still has a lot of growing to do. Also, your baby is gaining weight and that is smoothening out many of the wrinkles in his/ her skin.
By now your baby may be nearly 250 millimetres and weigh nearly 1,000 grams.
Otherwise healthy babies born this week have a 90 percent chance of survival without physical or neurological impairment - and the odds improve with each passing week.
Baby's bones developed fully
Twenty-nine weeks into your pregnancy (or 27 weeks after conception), your baby's bones are fully developed, but they are still soft and pliable. This week, your baby begins storing iron, calcium and phosphorus.
As your baby plumps up, the room in your womb will start to feel a little cramped. Instead of hard kicks from your little one, you will more likely feel jabs and pokes from elbows and knees.
By now your baby may be nearly 430 millimetres and weigh about 1,300 grams.
Brains grow bigger
Thirty weeks into your pregnancy (or 28 weeks after conception), your baby's brain is starting to look like one, taking on those characteristic grooves and indentations. The bigger and better brain is also starting to take on tasks previously delegated to other parts of the body, like temperature regulation.
During this week, your baby will start shedding lanugo, the downy soft body hair that has been keeping him/her warmth up to this point. That means by the time your baby is born, he/ she probably would be fuzzy no more.
By now your baby may be more than 450 millimetres long and weigh over 1,300 grams.
Sexual development continues
Thirty-one weeks into your pregnancy (or 29 weeks after conception), your baby is quickly approaching his/ her birth length.
You baby's sexual development continues. If your baby is a boy, his testicles are moving through the groin on their way into the scrotum. If your baby is a girl, her clitoris is now relatively prominent.
Also developing at an impressive speed is your baby's brain connections. He/ She is able to put all it to good use by processing information - tracking light and perceiving signals from all five senses.
By now your baby may be about 460 millimetres long (give and take) and weighs more than 1,300 grams.
Thirty-two weeks into your pregnancy (or 30 weeks after conception) and your baby is practicing his/ her moves like swallowing, breathing, kicking and sucking his/ her thumb.
Another change this week is that your baby's skin in no longer see-through. As more and more fat accumulates under the skin, it is finally opaque like yours!
Your baby's body is beginning to absorb vital minerals, such as iron and calcium. By this week, your baby's kicks and jabs may be more forceful.
By now your baby may be 480 millimetres and almost 1,800 grams.
Building immune system
Thirty-three weeks into your pregnancy (31 weeks after conception) and your baby is gaining weight fast; averaging out about 225 grams a week. Still your baby has lots of growing to do.
By this week, your amniotic fluid in your uterus is maxed out. This explains why your baby's pokes and kicks are sometimes extremely uncomfortable, as there is less fluid to cushion the blows.
Your baby is continuing to develop his/ her own immune system and the antibodies will be useful to protect him/ her from many germs.
By now your baby may be about 2,000 grams.
Baby's fingernails grow
Thirty-four weeks into your pregnancy (or 32 weeks after conception) and your baby's fingernails have reached his/ her fingertips.
By now your baby may be nearly 510 millimetres and weighs about 2250 grams.
Protective coating thickens
Thirty-five weeks into your pregnancy (or 33 weeks after conception) and your baby's body has become rounder. The pasty white coating that protects your baby's skin - the vernix caseosa - is getting thicker.
Brain development continues at a mind-boggling place, making your baby a little on the top-heavy side. By now, your baby may be in a heads-down position. That is a good thing, since it is easier on you if your baby's head exits first during delivery.
By now your baby may be about 510 millimetres and weighs about 2,475 grams.
Rapid weight gain begins
Thirty-six weeks into your pregnancy (or 34 weeks after conception), your baby is gaining weight rapidly, about 227 grams (0.5 pounds) a week for the next month. Your baby's systems are just about equipped for life on the outside.
The crowded conditions inside your uterus may make it harder for your baby to give you a punch, but you probably feel lots of stretches, rolls and wiggles.
By now your baby may be around 510 millimetres and weighs about 2,700 grams.
Baby is full-term
Thirty-seven weeks into your pregnancy (or 35 weeks after conception), your baby will be considered full-term. Your baby's organs are ready to function on their own.
Your baby is continuing to gain weight and fats are accumulating to form dimples on elbows, knees and shoulders with creases and folds in the next and wrists.
Your baby is practicing how to inhale and exhale, suck his/ her thumb, blink, pivot from side to side.
By now your baby weighs about 2,925 grams.
Baby develops a firm grasp
Thirty-eight weeks into your pregnancy (or 36 weeks after conception), your baby is developing a firm grasp.
Your baby's toenails have reached the tips of his/ her toes. Your baby's brain and nervous systems are working better every day. This developmental process will continue through childhood and adolescence.
By now your baby may be close to 340 millimetres about 3,150 grams.
Baby head moves into pelvis
Thirty-nine weeks into your pregnancy (37 weeks after conception), your baby's chest is becoming more prominent.
Your baby's head might have dropped into your pelvis which might make for easier breathing (and less heartburn), but could also make it harder for your to walk.
Your baby's brain is growing and developing up a storm and his/ her pink skin has turned white or whitish.
By now your baby may be nearly 510 to 530 millimetres and weighs about 3,150 to 3,600 grams.
Your due date arrives
Forty weeks into your pregnancy (38 weeks after conception), and you have reached the official end of your pregnancy!
Your baby is fully full term and weigh anywhere between 2,700 grams to 4,050 grams and measure between 480 millimetres to 560 millimetres. Remember, that healthy babies come in different sizes.
About half of all pregnancies proceed past the 40-week mark, although your practitioner will not let yours continue beyond 42 weeks.
Fewer than 5 percent of babies are born on their due date and about 50 percent will decide to overstay in Hotel Uterus. However, this does not mean that your baby is overdue. Sometimes, the due date was off.
An 'older' foetus will have longer nails, possibly longer hair and little or none of the baby fuzz (lanugo) at all. They are also more alert and open-eyed.
Just that all is well, your practitioner will monitor an overdue baby and advise you accordingly.