All 40 weeks·Trimester 1 weeks·Dashboard

Trimester 1 · Week 4 of 40

Week 4 of pregnancy: Implantation and a Positive Test

Implantation begins and hCG rises. When a home test becomes reliable, what a faint line means, spotting versus a period, and your first call to a doctor.

Last updated 28 September 2026

What's happening this week?

This is the week many people find out. The blastocyst implants in the womb lining and its outer cells start making hCG, so by the time your period is due a home test can turn positive. The embryo is still less than a millimetre across, and a scan usually shows nothing yet. If your test is positive, contact your doctor, ANM or local health centre to register the pregnancy, keep taking folic acid, and start thinking about where you would like your antenatal care.

Baby's development

Implantation starts about six to ten days after fertilisation, so for most people it happens during week 4. The blastocyst attaches to the thickened womb lining and its outer layer, the trophoblast, grows into it. This layer is the beginning of the placenta.

As it embeds, the trophoblast releases human chorionic gonadotropin (hCG) into your bloodstream. This is the hormone home urine tests detect, and in early pregnancy its level roughly doubles every two to three days, which is why a test that was negative a few days ago can now be positive. A darker or fainter line on a home test is not a way to judge how the pregnancy is progressing.

Inside the blastocyst, the inner cells arrange into a flat disc with two layers, known as the bilaminar disc. Two small fluid-filled spaces appear on either side of it: the amniotic cavity, which will later surround the baby, and the primary yolk sac, which supports the embryo in the early weeks before the placenta takes over. There is no heartbeat yet, because the heart has not begun to form. All of this happens within a structure smaller than a millimetre.

Your baby's size

About the size of a poppy seed

under 1 mm

< 0.1 in · Length

< 1 g

Weight

Week 4

Gestational age

At under 1 mm, the embryo is too small to measure. The poppy seed comparison gives only a sense of scale. A transvaginal scan usually shows nothing this week; a tiny pregnancy sac may appear at the very end of week 4 or in week 5, and the embryo itself later still.

Your body this week

Rising hCG and progesterone are behind the first signs. The most common is a missed period. Breasts may feel tender, fuller or tingly, and tiredness can arrive early. Some people pass urine more often, because hCG and increased blood flow to the kidneys both play a part.

Some people have light spotting or mild cramping around the time of implantation, which can be mistaken for a light period. Many have no spotting at all. There is no reliable way to tell implantation spotting from a period by its look alone, so a test is the better guide. Any bleeding after a positive test is worth a same-day call to your doctor, even though it often turns out to be harmless.

Finding out can bring joy, shock, worry or all three, whether the pregnancy was planned or not. It is normal to need time before telling anyone. If you are unsure about continuing the pregnancy, a doctor can talk through your options confidentially.

Symptoms you may notice

Every pregnancy is different - you may have some, all or none of these.

Missed period: The first sign for many people. If your cycles are irregular and you are unsure when a period is due, repeating a negative test a few days later can help, since hCG rises quickly.
Tender breasts: Hormones start preparing breast tissue from the beginning of pregnancy. A soft, supportive bra can ease the soreness.
Tiredness: Progesterone can make you unusually sleepy. Earlier nights and short rests help, and it often lasts through the first trimester.
Light spotting and mild cramps: These can happen around implantation. Note how much you are losing, and contact your doctor the same day about any bleeding after a positive test.
Passing urine more often: Caused by hCG and extra blood flow to the kidneys. Burning, stinging or cloudy urine is different and may be an infection.

Medical care and tests

Home urine pregnancy test
Routine
From the day your period is due
Home tests detect hCG in urine and are most reliable from the day of the missed period. In India a free card test is also available at sub-centres and health centres. A faint line usually still means hCG has been found; because hCG roughly doubles every two to three days early on, repeating the test after two or three days should give a clearer line.
Registering the pregnancy
Routine
As soon as pregnancy is suspected, within 12 weeks
Indian guidance asks for registration and a first check-up as early as possible, and within 12 weeks. You receive a Mother and Child Protection (MCP) card that records your visits, tests and vaccines. In the UK you can refer yourself to maternity services, and in the US you contact a provider.
Early scan
Only if needed
Not usually this week
A scan in week 4 usually shows nothing, so it is rarely helpful yet. Your doctor may arrange one sooner if you have pain or bleeding, to check that the pregnancy is in the womb.
Folic acid
Routine
Daily until the end of week 12
Continue 400 micrograms a day. If you had not started before conception, start now; the neural tube forms over the next two weeks.

Government health centres provide antenatal check-ups, tests and supplements free of charge, so registering early gives you access to the national programme. Private care is also fine, as long as you have a plan for your first visit.

Nutrition this week

Small, regular meals every few hours can keep energy steadier as early pregnancy hormones rise, and may make queasiness easier to handle if it starts.
Indian dietary guidelines suggest more than 2 litres of fluids a day. Water, buttermilk, nimbu pani, coconut water and thin dal all count.
Keep easy snacks within reach: roasted chana, a banana, a handful of peanuts or a small bowl of poha.
Keep tea and coffee modest and away from meals while you get used to the idea of a caffeine limit.
Continue folate-rich foods such as greens and dals alongside your folic acid tablet.

Movement and exercise

Pregnancy guidelines encourage regular moderate activity for most women, so carry on with the walking, yoga or other exercise you already do.
Stop and contact your doctor if exercise brings on bleeding, dizziness or pain.
Switch from hot yoga or intense sessions in the heat to cooler, moderate workouts, and drink water before and after exercise.
If you have not been active, walking is a good place to start: begin with short walks and build up gradually as you feel able. You do not need to wait until the second trimester.

Things to keep in mind

Write down the date of your last period and the date of your positive test, because you will be asked for both.
Decide who to tell and when. Many people wait until after the first scan, but there is no rule.
If your work involves chemicals, radiation or heavy lifting, speak to your employer about adjustments now.
Save the phone number of your nearest hospital's emergency or labour ward.
Remember that nothing is likely to show on a scan for another week or two, so a quiet first visit is normal.

When to contact your doctor

Call your doctor or midwife the same day if you have

Any bleeding or spotting after a positive test.
Cramping that is stronger than period pain or does not settle.
Burning or stinging when passing urine.
Feeling feverish, with a temperature of 38°C or more.
Vomiting so often that you cannot keep fluids down.

Get emergency care now if you have

Heavy bleeding that soaks a pad in an hour or less, or passing clots.
One-sided lower tummy pain with dizziness, fainting or shoulder-tip pain.
Severe tummy pain.
Fainting or collapse.

In India, call 108 for an ambulance, 102 for free transport for pregnant women in many states, or 112 for any emergency - or go straight to your nearest hospital with a maternity unit.

If something feels wrong, trust that feeling and call - you never need to wait for your next appointment.

Questions to ask your doctor

When would you like to see me for the first visit, and which tests will be done?
How do I register the pregnancy and get an MCP card?
Do I need an early scan, and if so when?
Should I stay on my current folic acid tablet?
Which of my regular medicines should I continue?

Week 4 checklist

Take a home test on or after the day your period is due
Note the date of your last period
Call your doctor, ANM or health centre to register
Keep taking folic acid
Save the nearest hospital's number

Frequently asked questions

Is a faint line on a pregnancy test a positive result?

A faint line usually means the test has picked up some hCG, often because you are testing early and levels are still low. As hCG roughly doubles every two to three days in early pregnancy, repeating the test after two or three days should give a darker line. If results stay unclear, or you have pain or bleeding, see your doctor, who can arrange a blood test.

How can I tell implantation spotting from a period?

It can be hard. Spotting around implantation, when it happens, tends to be light and brief, whereas a period usually builds up and lasts several days. But neither the colour nor the amount is a reliable test. A pregnancy test from the day your period is due is the clearest guide, and any bleeding after a positive test should be discussed with your doctor the same day.

When should I see a doctor after a positive test?

Contact your doctor, ANM or health centre soon after a positive result, so the pregnancy can be registered and the first visit planned. Indian guidance recommends the first check-up as early as possible and within 12 weeks. Seek help the same day if you have bleeding, pain or vomiting that stops you keeping fluids down.

This page is general information, not a diagnosis or a substitute for care from your own doctor or midwife. Pregnancies differ, and timings here are typical ranges rather than deadlines. Read our medical disclaimer and editorial policy.