How Many Weeks Pregnant Am I? Weeks, Months and Due Date Explained

"You're 10 SA." You leave the surgery with that figure in your head, your app says 8 weeks, and your mother-in-law talks about the second month. Everyone is right, more or less: they are just counting differently. Working out how many weeks pregnant you are is not complicated once you know where each count starts.

Here is how to convert weeks into months, estimate your due date, and understand why that date can move.

SA, weeks since conception, months: why three ways of counting?

Because nobody knows the exact day of fertilisation. Health professionals therefore start from a date most women know: the first day of their last period, often shortened to LMP (last menstrual period), or DDR in French-language medical notes in Morocco.

• Weeks of amenorrhoea (SA, semaines d'aménorrhée) are counted from the LMP. This is the unit used by doctors, midwives, sonographers and your medical notes in Morocco. It is exactly what English-speaking doctors simply call "weeks pregnant" or gestational age.
• Weeks of pregnancy counted from conception (SG, semaines de grossesse, in French) start at fertilisation, which happens about two weeks after the LMP in a 28-day cycle. They are therefore roughly two weeks fewer than SA.
• Months are still the language of family and many apps, but they are vague, because a month is not a whole number of weeks.

Tommy's, a specialist UK charity, puts it this way: your period and ovulation count as the first two weeks of pregnancy. A baby born at 40 weeks has therefore been developing for about 38 weeks.

How do you work out how many weeks pregnant you are?

Count the number of days since the first day of your last period, then divide by 7. The result is written in weeks and days.

Example. Your last period started on Saturday 10 January 2026. On 25 March 2026, 74 days have passed: you are 10 weeks + 4 days (10 SA + 4 days), or 8 weeks and 4 days since conception. In other words, 10 full weeks have passed and you are in your 11th week.

This calculation assumes a cycle of about 28 days, with ovulation around day 14. It gives a good first estimate while you wait for the scan.

How do you work out your due date?

This is where countries differ, and it is the source of a lot of confusion.

With an LMP of 10 January 2026, the due date falls on 17 October 2026 at 40 weeks, or 24 October 2026 at 41 weeks. A week's difference, which explains why two online calculators don't give the same result. Geneva University Hospitals actually show "40-41 SA" on their pregnancy calendar.

In Morocco, many doctors were trained in the French system, so simply ask your gynaecologist or midwife which date they use: it is the one that will appear in your notes and on your certificates.

Conversion table: weeks, months and trimesters

Pregnancy months are counted from conception. Based on the French reference of 9 months for 41 weeks, here are the usual markers, rounded.

Trimesters are not exactly three months of round weeks. The Quebec guide Mieux vivre avec notre enfant splits pregnancy into three blocks of about 14 weeks: in practice, the first trimester runs to 14-15 weeks, the second to 28 weeks and the third until the birth. To find out what happens at each stage, see our article on pregnancy week by week.

What if my cycles are irregular or long?

Counting from the LMP assumes ovulation around day 14. With 35-day cycles, ovulation happens later, and the pregnancy is actually less advanced than the calculation suggests. Reliable calculators, such as Tommy's, ask for your average cycle length to correct the estimate.

If you don't know the date of your last period, because your cycles are irregular, you were breastfeeding or you have just stopped contraception, the calculation isn't possible. That is fine: the scan takes care of it. Our article on ovulation and fertile days explains how a cycle works.

Why can the scan change the date?

Because it measures the embryo itself, instead of assuming a date of ovulation. In early pregnancy, all embryos grow at roughly the same rate, so their size gives their age precisely.

• According to the Larousse encyclopaedia, measuring the embryo between 8 and 12 weeks (crown-rump length) gives the due date to within 3 days.
• The Quebec guide Mieux vivre says a scan done before 20 weeks estimates the due date to within 7 to 10 days.
• In the UK, the date calculated at the dating scan becomes the reference date for the rest of the pregnancy.

If the sonographer corrects your due date, the new date is the one that counts. It does not mean your baby is "growing too fast" or "not fast enough": it is the starting point that has been adjusted. The role of each examination is explained in our article on pregnancy scans.

The due date: an approximate date

The estimated due date is just that, an estimate. Only about 4 babies in 100 are born on that day, according to Tommy's. A pregnancy normally lasts from 37 to 42 weeks, and most babies are born between 39 and 41 weeks, according to the Quebec guide. A birth is called term from 37 weeks, and premature before that.

It is better to think in terms of a "window" than an exact day. Once you are past the 8th month, keep your hospital bag ready by the door, and reread the signs that labour is starting.

From the shop

As your due date approaches

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In Morocco: the date that counts for your paperwork

The estimated due date is also an administrative marker. In the private sector, maternity leave lasts 14 weeks, in principle split into 7 weeks before the birth and 7 weeks after (Labour Code, article 152). The split can be adjusted on medical advice, and if your baby comes early, the antenatal weeks not taken are added to your leave after the birth.

Depending on whether your doctor uses 40 or 41 weeks, the theoretical start of your leave can therefore shift by a week. Clear this up before talking to your employer. All the details, including CNSS (social security) benefits, are in our article on maternity leave in Morocco.