A guide to your pregnancy week by week: Weeks 1-4.

Whether you’re still in the trying stages or have just learned that you’re pregnant, knowing what to expect each week can be essential.

Dr Sonya Jessup is an experienced Fertility Specialist and Gynaecologist at Demeter Fertility. She told Mamamia about the early stages of pregnancy week by week.

The overview.

“The most important thing about the first four weeks of pregnancy is that people mean different things by it,” Dr Jessup says.

“When we are talking about the first week of pregnancy or the fifth or sixth week, by definition we take it from the first day of last menstrual period.”

“By the time you miss your period, by definition you are four weeks pregnant at that point even though you obviously only release the egg only two weeks into that cycle… But we relate everything back to that last period because from days gone by, that’s all we could go by.”

Dr Jessup also explains that a pregnant woman who is described as eight weeks pregnant is only actually six weeks on from when they released an egg and the sperm and the egg got together, but by definition medically, it will be always called eight weeks.

"Patients sometimes get confused because they will say, 'I am two weeks pregnant' because they’ll know when they ovulated and they’ll know they could only have been pregnant for those two weeks," says Dr Jessup.

"But by definition, it always goes back to the last period in a regular cycle.

"If you have got a normal cycle by the time you realise your period is a day late and you do a pregnancy test, you are four weeks pregnant at that time."

When the dates don't add up.

If you’ve got an irregular cycle and your periods come five or six weeks apart the dates of the last menstrual period (LMP) and conception might not match, which is where timing can be tricky.

"In those people when you work out how pregnant you are, what we tend to do is see how far they are on scans and push it back and give a LMP that is closer to when they would have actually ovulated," says Dr Jessup.

Listen: You're pregnant, now what? Post continues... 

Conceiving.

Dr Jessup explains an egg is released in a normal 28-day cycle on day 14.

"Then the egg has 12 hours in which it can fertilised. You have to have sex either exactly at that time or you have to have sex a few days before so there’s sperm around waiting for the egg," she says.

"Then the sperm goes round the outside of the egg, they create a reaction to let one sperm in and that’s when fertilisation happens. That happens around day 14 or 15.

"For the next few days is those cells begin to multiply and divide," she says.

"By the time you get to five days after you have released the egg there’s already about 100 cells in that little pregnancy creating a ring of cells around the outside and a little lump where the baby is going to be in the middle," Jessup says.

"By the time you have actually missed your period  - you’re four weeks pregnant - that little round circle of cells is beginning to implant into the wall of the uterus."

Then what happens?

Dr Jessup says you can’t see the sac on an ultrasound scan until about five weeks.

"A week after you have missed your period you can begin to see a little sack [on ultrasound scans]," she said.

"By six weeks - two weeks after you missed your period - you can begin to see a sac with a little circle in the middle called a yolk sac and that’s when you can begin to see a little two to three millimetre little foetal pole with the beginning of a foetal heart rate," says Dr Jessup.

Do you need to avoid anything?

"I tell patients once they realise they are pregnant it’s important not to do anything toxic, don’t find yourself spraying the oven, or putting polyurethane on the floors, or painting your house because that’s the time all the organs are forming," says Dr Jessup.

"You don’t want the pregnancy to be exposed to any toxins – particularly in that time." (Post continues after audio.)

Try not to worry too much.

Dr Jessup says some people worry if they have been going out partying and then realise they are pregnant. She says the first two weeks are an "all or nothing phase".

"It doesn’t seem to affect the baby too much at that time. Once you have realised you are pregnant, the baby is by then four weeks by definition, or two weeks since fertilisation occurred. But we call it four weeks.

"You can pretty much rest assured you can live a normal life until you realise you're pregnant. At that point, you realise that’s when you can cut down on coffee and stop drinking and all the rest of it and that’s fine," says Dr Jessup.

Dr Sonya Jessup, BHB MBChB MRM FRANZCOG

Dr Jessup is an experienced Fertility Specialist and Gynaecologist at Demeter Fertility, who has studied and worked in Reproductive Medicine for the past decade.  She provides a thorough and personalised assessment of a patient’s situation and can quickly organise and perform any procedures, tests or surgery they may need. Dr Jessup has been through IVF herself and works to create a happy, low stress, sustainable approach to fertility treatments.