How are the weeks counted?

The Nine Months Calendar displays the actual size of the fetus during pregnancy, week by week. But how are the weeks counted, from the beginning of the menstrual cycle or until conception has taken place?

It is good to know a thing or two about the female body and its cycles before we answer this question.

The first day of the menstrual cycle starts on the first day a woman has her period. The period usually lasts 3 to 7 days. The menstrual cycle can vary in length but is generally between 23 and 35 days.

Ovulation takes place halfway through this cycle; the maturation of the egg and the jumping of the egg. When fertilization takes place, the 23 maternal chromosomes and the 23 paternal chromosomes attach. At that moment, they create the first edition of the embryo. The embryo is now said to be one day old, while the pregnancy already lasted two weeks.

From this day on, cell division begins. Every day, each gene makes copies of itself. Thus, the embryo starts to grow.

Pregnant woman in front of the Nine Months Calendar

After a few weeks, the brain, heart, and central nervous system begin to form. The mother’s body also begins to adapt to the new life in the womb. Meanwhile, the expectant mother usually knows they are pregnant. Whether or not because of the hormonal changes (such as nausea), or because of the missing menstrual period.

These beautiful changes will continue for a while. The eyes, nose, organs, nerves and millions of other cells will be formed. This process continues until a baby is born, 40 weeks after the first day of the last menstrual cycle. At that moment, the baby is factually only 38 weeks old.

This calendar shows the growth of an average Dutch child. While every child will have a different growth curve, this calendar shows an average. Boy or girl? That doesn’t matter. The illustrations do not identify gender.

week 21 Nine Months Calendar

The calendar

A true-sized pregnancy calendar, no longer puzzled with avocados, cauliflowers and other vegetables. The Nine Months Calendar shows the size of the baby in the belly every week.

Details ›
We are on vacation from June 9-27. Your order will ship the week of June 28, when we return.