Why Pregnancy Is Counted in Weeks — Not Months

If you ask a medical professional how far along a pregnancy is, they will almost never answer in months. The medical world uses weeks because foetal development happens so rapidly that a month is simply too broad a measurement. A baby at 24 weeks is at a vastly different developmental stage than a baby at 27 weeks, even though both are technically in their "sixth month."

Furthermore, the 40 weeks of pregnancy are counted from the first day of the person's last menstrual period — not from the date of conception. This is because conception dates are often difficult to pinpoint accurately, whereas a last period is usually a known date.

Due dates are just estimates. The "40 weeks" mark is simply an average. It is perfectly normal and healthy for a baby to be born anywhere between 37 weeks and 42 weeks. In fact, only about 5% of babies arrive on their exact calculated due date.

The First Few Days — What the Numbers Mean

The numbers in a newborn's life can be surprising. For example, almost all healthy newborns lose weight in the first few days of life — often up to 10% of their birth weight — as they shed excess fluid. They typically regain this weight by two weeks of age.

Understanding a baby's stomach capacity explains their feeding and sleeping patterns. On day one, a newborn's stomach is only the size of a cherry, holding just 5–7ml. Because it is so small, breast milk or formula empties quickly, which is why newborns need to feed 8 to 12 times a day. Consequently, while they sleep 16 to 17 hours a day in total, it happens in short, chaotic bursts between feeds.