Why pregnancy lasts 40 weeks, not 38

Doctors don't date a pregnancy from conception. They count from the first day of your last period, because very few women know exactly when they conceived, while most remember when their period started.

That means in the first two 'weeks' of pregnancy there is no baby yet. Your body is getting ready: an egg is maturing and the lining of the womb is thickening. Ovulation and conception usually happen around the end of week two, and by the start of week four the embryo is settling into the wall of the womb.

So there is no real contradiction. About 38 weeks pass from conception to birth, yet your exchange card and scan reports will show 40 'obstetric' weeks. If your cycle is long or irregular, the doctor will confirm your dates at the 11–13 week scan, and that date becomes the one that counts.

Today's small step: save the first day of your last period in your phone. You will be asked for it at your first polyclinic visit, and our due date calculator needs it too.