16 weeks pregnant

How big is your baby at 16 weeks?

About the size of a avocado — around 11.6 cm (4.6 in) long head to bottom, according to the NHS, weighing roughly 144 g (5 oz).

The NHS notes your baby is about to have another growth spurt, and that you will probably have put on some weight over the past few weeks too.

A note on the numbers: up to week 19 the NHS measures crown to rump — head to bottom, because the legs are curled. At week 20 it switches to head to heel, which is why the length appears to leap 10 cm in a week. Weights here are the median from the WHO fetal growth charts, which begin at 14 weeks; half of babies weigh more, half less.

What's developing this week

Your baby is starting to pull faces. Any smiling or frowning is completely random, because there is no muscle control yet — the expressions are the nervous system testing its wiring, not a mood.

That same development is what now lets your baby move the arms and legs with intent. The hands can form fists, and the NHS says they may start punching around inside you. It also says you might feel kicking from week 17 onwards — the earliest end of a wide range, not a promise.

Underneath, the work ACOG describes for weeks 13 to 16 is finishing: long bones hardening, toenails formed, hearing developing, and the lungs building tissue they will need much later.

What you might feel

A growth spurt in your baby usually means a visible change in you. Many people find this is the fortnight their own clothes stop working.

Constipation

Common now, and miserable: it can leave you bloated, nauseated and aching. The NHS suggests high-fibre foods such as wholemeal bread, fruit and vegetables, beans and lentils, regular exercise and plenty of water. It also says to avoid iron supplements if they are the cause — but talk to your doctor or midwife before stopping any medication, because iron in pregnancy is usually prescribed for a reason.

The rest of the list is unchanged: bleeding gums, round ligament pain, headaches, nosebleeds, bloating, heartburn, sore breasts, leg cramps, dizziness, swollen hands and feet and urine infections. The NHS adds a line worth taking seriously — if you feel unwell and it is getting you down, say so.

Is 16 weeks pregnant 3 months or 4 months?

Week 16 is in month 4, its third week.

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 — "19 weeks and 3 days" — because a few days change what is offered. The anomaly scan window is 18+0 to 20+6 weeks, not "about five months".

What's happening at your appointments around now

This is the first named appointment of the second trimester in the UK, and more substantial than it looks on the schedule.

The midwife will weigh you, ask how you are getting on, and go through the blood tests taken at your booking appointment. That includes the screening offered for three infectious diseases — HIV, hepatitis B and syphilis. If anything has been picked up, this is where you discuss treatment and reducing the chance of passing an infection to your baby. All three are manageable, and finding them early is what changes the outcome.

Your blood pressure is checked and you will be asked for a urine sample, tested for protein — an early sign of pre-eclampsia. NICE requires both at every routine face-to-face appointment from here on. There is also now a routine carbon monoxide breath test at midwife appointments.

You may hear your baby's heartbeat for the first time through a handheld doppler. It is not guaranteed — position and dates both affect it — and not hearing it at 16 weeks is not a warning sign.

The whooping cough vaccine

Week 16 is the earliest you can have it. The NHS usually offers it around week 20, at about the time of the mid-pregnancy scan, but you can have it from 16 weeks and should have it before 32. The point is not to protect you: the antibodies cross the placenta and protect your baby in the first weeks of life, before their own vaccination at 8 weeks old.

Is it safe to fly at 16 weeks pregnant?

Yes, in a straightforward pregnancy — and you are inside the window everyone recommends. 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 none that flying causes miscarriage, early labour or your waters to break. ACOG names 14 to 28 weeks as the best time to travel; the NHS names mid-pregnancy.

If you are flying soon after a 16-week appointment, ask the midwife directly. They know your blood pressure, your bloods and your history, and the RCOG says an individual clot risk assessment is something you can ask for.

The risk that is real: blood clots

Pregnancy raises deep vein thrombosis (DVT) risk, and that does not ease in the second trimester. ACOG puts it plainly: any travel of four hours or more 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

• Airline cut-offs are usually 37 weeks for one baby, 32 for twins, so nothing restricts you yet — but from 28 weeks most airlines want a letter from your midwife or doctor, which the NHS warns can cost money and take weeks.
• Check what maternity and emergency care exists at your destination, and tell your insurer you are pregnant so that cover includes pregnancy-related care.
• Some travel vaccines and anti-malarial tablets are unsuitable in pregnancy; travel to Zika areas is not recommended.

When to call your midwife or doctor

The second trimester is the quietest stretch of pregnancy for most people, which is exactly why it is worth knowing what is not ordinary. Maternity units are staffed around the clock.

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:

• Heavy vaginal bleeding, or any bleeding with severe abdominal pain
• Abdominal pain you cannot concentrate through
• Fainting or collapse, chest pain, or sudden breathlessness
• A severe headache painkillers do not touch, blurred vision or flashing lights, pain below your ribs, or sudden swelling of your face, hands or feet — the NHS lists these as pre-eclampsia symptoms, most likely from 20 weeks, needing immediate assessment

Call your midwife or maternity unit today, whatever the hour

• Any vaginal bleeding, however light
• A gush or a steady trickle of fluid from your vagina
• Regular tightenings, or persistent low backache and pelvic pressure
• Discharge that changes colour or texture, smells strongly, or brings itching or soreness
• Itching without a rash, especially at night or on your palms and soles — the NHS asks for this to be checked, because it can be intrahepatic cholestasis of pregnancy
• A temperature, feeling generally unwell, or burning when you pee
• Not feeling movement yet is not on this list. At 16 weeks almost nobody has.

You do not need to be certain something is wrong. That is what the number is for.