Before we finally welcome our newborn baby into this world, we parents go through months of waiting. It starts from the time we get a positive pregnancy test result either by a pregnancy test home kit or from a hospital laboratory. From then on, our emotions get higher and higher everyday as we’re constantly thrilled by new surprises that come along the way. One cannot fathom the joy of seeing our baby for the first time on ultrasound. The feeling is indescribable! I remember back then while I was having my radiology rotation during my internship, some couples cry for joy, while others can’t keep themselves from smiling to see their baby on the ultrasound monitor. At about 4-5 months, comes the excitement of knowing the baby’s gender. Then the fun of thinking about the best name for the baby begins, as well as predicting as to whom the baby might probably look similar to. All these surprises make the seemingly long months of waiting appear shorter and happier than we first imagined it to be. But have you ever wondered how the baby grew inside the womb, considering the time it has spent inside?

Structural growth of the baby is divided into 2 developmental periods. The first is called the embryonic period which begins at the third week of fertilization up to the 3rd month of pregnancy. [1] This stage is crucial since it is during this time that most of the vital parts of the baby’s body are beginning to form. Some factors can greatly alter this biological process, including maternal infections and medications taken by the mother, the effects of which are usually irreversible. Hence, the mother is strongly advised to avoid taking medications unless prescribed by a physician and avoid contracting illnesses such as German measles.

Next, is the fetal period, which occurs from the 3rd month of pregnancy and onwards. During this time, growth and maturation of structures that were formed during the embryonic period takes place. [2] At the beginning of this period, the baby is now 3 inches long, weighs about 28 grams and has slightly webbed fingers. Transparent skin and nails have developed, and hair rudiments appear. [1] At 4 months, the baby can already fit in a tea cup. Gender can be determined at this stage. At 5 months, midpoint of pregnancy, the baby now weighs 300 grams. The fetal skin has become less transparent. [1] At 6 months, skin wrinkling smoothens out as fat deposition begins.[2] The baby’s lung is starting to develop. A baby born during this period will attempt to breathe, but will likely die because air sacs in the lung that provide gas exchange have not yet completely formed. [1] At 7 months, the baby can follow a light and can open and close his eyes. [2] At about 8 months, the baby is now 11 inches, weighing 1800 grams. [1] This is the time when the baby’s lung is fully developed. Thus, babies born at this time have increased chances of survival. At 9 months and onwards, average baby’s length is more than 17 inches long and weighs more than 2500 grams.[1] During this time, the sizes of babies vary widely.

Now that we know at what month of pregnancy a newborn will likely survive, it is necessary for us to calculate the expected date of delivery. When you want to know your due date or expected date of delivery, just add 7 days and subtract 3 months to the first day of your last menstrual period. For example, if the first day of your last menstrual period was May 20, 2008, subtract 3 months from May and add 7 days to 20. Therefore, the due date is February 27, 2009. The due date roughly coincides to 40 wks of gestation. Since 37-42 weeks is the normal gestational age for delivery, delivering the baby 3 weeks before and 2 weeks after the due date is still considered normal and term. [1] But when you want to know how old the baby growing inside you is, (called the gestational age), just remember the first day of your last menstrual period and count from there. However, the prerequisite for applying these methods of calculation is that the mother should have regular monthly menstrual periods prior to pregnancy.

The formation of a baby from the union of specialized reproductive cells is a miracle of nature. A lot of unseen, unappreciated processes have to occur before we finally see our bundle of joy.


[1] Cunningham FG, Fetal Growth and Development. Williams Obstetrics 21st edition.
[2] What Your Baby Looks Like This Week. Fetal Development.