How many weeks pregnant are you?

Dear mum, you are reasonably sure by now that you are pregnant, and so congratulations. Despite being sure of the pregnancy, it’s not always easy to answer the question, “How many weeks pregnant?”, is it?

Not to worry, we’re here to help. There are actually two ways of answering that question, and both ways are in use today.

The first is a measure that’s been passed down the generations, to your grandmother and then mother, and it is called gestational age. It needs simple observations and some ‘mummy mathematics’.
The second measure is fetal age, which needs sophisticated technology.

Gestational age
Ovulation and fertilisation have no outward signs and cannot be dated. Day-1 of the last menstrual period (LMP) occurs about 2 weeks before ovulation. Weeks counted from that LMP date give your gestational “weeks pregnant” number. It is a good informal measure, with a fun bonus; the idea that you were already 2 weeks “pregnant” at ovulation!
Gestational age (once the only measure used), is still used to calculate due date informally. Gestational term is 40 weeks (280 days) from LMP.

Fetal age, or ovulation system age
The fetal age is the actual age of your baby—that is, how many weeks since the egg and sperm came together. It is calculated using modern technology such as ultrasound or the far more accurate ovulation testing, whereas gestational age is based on a guess, that LMP is 2 weeks before ovulation. For menstrual cycles that are much shorter or much longer than the average 28 days, fetal age is more accurate than gestational age.

Trimesters
Pregnancy is usually divided into 3 “trimesters” of around 12 weeks each. The division is logical because it represents 3 fairly different stages of pregnancy in terms of the experiences that you as a pregnant woman have, and the development of your baby.

How many weeks will your pregnancy last?

To calculate due date, you need to know how long your pregnancy will last.
The traditional method of estimating the due date of a baby, called Naegele's rule, is to add 7 days to the 1st day of your last menstrual period, then 9 months to the month.
So, if your last period started on November 18, then your due date is 40 weeks away, on August 24. Naegele's rule assumes a 28-day menstrual cycle with ovulation and fertilisation on day 14; so gestation is 38 weeks from the day of ovulation.
However, as accurate as these numbers try to be, they are only guidelines. Only 3–5% of babies actually arrive on their due dates.
The most common time for babies to arrive is 40 weeks and 3 days. Statistically, you are likelier to be overdue than early.
Most professionals argue that babies can be born perfectly healthy at anything from 37 to 42 weeks after LMP.