4 weeks pregnant

How big is your baby at 4 weeks?

About the size of a poppy seed — the NHS puts the embryo at around 2 mm (0.08 in) long. Nothing yet that would look like a baby, but a complete life-support system under construction: an amniotic sac of cushioning fluid, a tiny yolk sac supplying everything the embryo needs, and an outer layer of cells that will become the placenta.

A note on the numbers: in the first trimester babies are measured crown to rump — head to bottom, not head to heel, because the legs are curled. Weight is not reliably estimated this early, so any site quoting grams or ounces before about 14 weeks is guessing.

What's developing this week

Week 4 is implantation week. The ball of cells burrows into the lining of your womb and starts producing hCG, the hormone a pregnancy test detects — the most important event of the week, and it happens entirely without your knowledge.

Inside, the embryo has separated into the layers that become every tissue in the body: skin and nervous system, bone and muscle and heart, gut and lungs. The neural tube, which becomes the brain and spinal cord, is about to start closing — which is why folic acid matters now rather than later. The NHS advises 400 micrograms daily through the first 12 weeks, plus 10 micrograms of vitamin D, and a prescribed 5 mg folic acid dose for some people. Avoid vitamin A supplements, cod liver oil and liver.

What you might feel

Most likely, nothing. The NHS says plainly that in the first four weeks you probably will not notice any symptoms, and that the first sign for most people is a period that does not arrive.

Some people notice tender breasts, mild cramping like a period that never quite starts, a metallic taste, tiredness, or light spotting as the embryo implants. Any of these can also be premenstrual, which is exactly why this week is impossible to read.

An absence of symptoms is not a bad sign. Hormone levels rise at different rates in different people, and feeling nothing at 4 weeks predicts nothing.

Is 4 weeks pregnant 1 month or 2 months?

Week 4 is the final week of month 1. At the end of this week you are one month pregnant, and week 5 starts month 2.

There is no official weeks-to-months map. A calendar month averages about 4.3 weeks, not 4, so 40 weeks is a little over nine months — and two apps can disagree about which month you are in. The most widely used mapping:

• Month 1 — weeks 1 to 4
• Month 2 — weeks 5 to 8
• Month 3 — weeks 9 to 13
• Month 4 — weeks 14 to 17
• Month 5 — weeks 18 to 22
• Month 6 — weeks 23 to 27
• Month 7 — weeks 28 to 31
• Month 8 — weeks 32 to 35
• Month 9 — weeks 36 to 40

Your maternity team will not use months. Everything is booked by week and day — "11 weeks and 3 days" — because a few days change what is offered.

The confusing part at 4 weeks: pregnancy is dated from the first day of your last period, not from conception. So you were counted as pregnant for about two weeks before you were. Conception happened roughly two weeks ago; you are four weeks pregnant. Both are true.

Miscarriage risk at 4 weeks

This is the highest-risk week of the ten covered here, and also the week when a loss is most likely to pass unrecognised as a late or heavy period.

Tommy's estimates that early miscarriage — loss before 12 weeks — happens in 10 to 20 out of every 100 pregnancies, and that most miscarriages happen in those first 12 weeks. The RCOG describes first-trimester miscarriage as common, around one in five. The true figure at 4 weeks is higher than either, because many very early losses happen before anyone knows to count them.

You will find week-by-week miscarriage percentage tables all over the internet. Tommy's, which funds the UK's national miscarriage research centre, says openly that beyond the well-evidenced drop after 12 weeks, reliable week-by-week risk research does not really exist. We give you what is evidenced instead.

What is evidenced: risk falls sharply once a heartbeat is seen at 6 to 7 weeks, and sharply again after 12 weeks. You are before both markers. That is a statement about a curve, not a prediction about you.

The most common cause of early miscarriage is a chromosomal problem that happens by chance. Not lifting something, not a glass of wine before you knew, not stress or exercise. Tommy's states it directly: miscarriages very rarely happen because of something you did or did not do.

Is it safe to fly at 4 weeks pregnant?

Yes, in a straightforward pregnancy. The RCOG is unambiguous: occasional air travel is not harmful in an uncomplicated pregnancy. There is no evidence that cabin pressure or humidity harm you or the baby, and no evidence that flying causes miscarriage, early labour or your waters to break.

The RCOG adds a nuance worth understanding now: most obstetric emergencies happen in the first and third trimesters. Flying does not cause them, but it can put you a long way from care. The RCOG also notes that miscarriage in the first three months is common whether you fly or not — so a loss on holiday was not caused by the flight, however it feels afterwards.

The risk that is real: blood clots

Pregnancy raises deep vein thrombosis (DVT) risk from the earliest weeks. ACOG puts it plainly: any travel of four hours or more — car, train, bus or plane — doubles DVT risk, and pregnancy adds to that. Under four hours the RCOG says you are unlikely to need special measures. Over four hours:

• Loose clothing, comfortable shoes, an aisle seat, regular walks about the cabin
• In-seat exercises roughly every 30 minutes
• Water steadily; cut down on caffeine
• Graduated compression stockings — not ordinary flight socks, and sized for you by a midwife or doctor

With other clot risk factors you may be advised to have heparin injections around the flight.

Practical points

• No airline restricts travel this early — cut-offs sit around 36 to 37 weeks for one baby, 32 for twins.
• Check what maternity and emergency care exists at your destination.
• Tell your insurer you are pregnant; cover should include pregnancy-related care.
• Some travel vaccines and anti-malarial tablets are unsuitable in pregnancy; travel to Zika areas is not recommended.

The NHS notes many people prefer not to travel in the first 12 weeks because of nausea and exhaustion. ACOG names 14 to 28 weeks as the best time. Neither says the first trimester is unsafe.

When to call your midwife or doctor

Early pregnancy is full of sensations that turn out to be nothing. These are the ones worth a phone call.

Get emergency help now

Call your local emergency number (999 in the UK and Ireland, 911 in the US and Canada, 000 in Australia, 111 in New Zealand) or go to an emergency department if you have bleeding with any of the following, which the NHS lists as needing immediate assessment:

• Severe abdominal pain you cannot concentrate through
• Pain in the tip of your shoulder
• Feeling sick, faint or dizzy, or losing consciousness
• Heavy bleeding that soaks through a pad quickly

Sudden sharp abdominal pain with dizziness, faintness or looking very pale can indicate a ruptured ectopic pregnancy, needing same-day surgery. Around 1 in every 90 UK pregnancies is ectopic, and the NHS notes symptoms usually appear between weeks 4 and 12 — where you are now.

Call your midwife, early pregnancy unit or health line today

• Light bleeding or spotting, with mild or no pain
• One-sided pain low in your abdomen or pelvis
• Brown, watery discharge that is new for you
• Pain when you pee or open your bowels
• You cannot keep food or fluid down for 24 hours, your urine is very dark or absent for 8 hours, you feel severely weak or faint, or you are vomiting blood or losing weight — the NHS triggers for pregnancy-sickness help, which is treatable

You do not need a booking appointment first, and you do not need to be certain something is wrong.