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Pregnancy weeks:

Week 15 of pregnancy: Their first tiny hairs develop

In week 15, you’re three weeks into the fourth month of your pregnancy and are now in the second trimester.

Size of your baby in week 15

By now your baby is 9.5 to 10 cm in size, so about as big as an orange, and weighs about 70 grams. Their heart is fully developed and now just needs to grow. You’ll be able to hear it beating in an ultrasound, and hearing that for the first time is sure to be a special moment for you and your partner – it makes the life growing inside you feel so much more real.

Your baby’s genitals are also fully formed and can be seen in an ultrasound. The doctor previously couldn’t tell you which sex they were, as a boy’s testicles look very similar to a girl’s protruding labia. In week 15, your doctor can be very confident of the sex, but don’t take what they say as final just yet, as they may discover otherwise in the next few weeks.

From now, on your doctor will use the ultrasound scanner to measure the biparietal diameter, which is a measure of the foetus’s head. BPD is a key value, as it’s the most effective way to determine your baby’s development and growth.

The placenta will be supplying your baby around the clock with the nutrients and oxygen they need – and they need quite a lot in order to develop properly and grow into a healthy human being. This means that the placenta plays a key role in their breathing and in removing the substances they excrete. As their lungs aren’t fully developed and won’t start working until they’re born, they get their oxygen from the blood flowing through the umbilical cord straight to their body.

Your baby’s development

In week 15, your baby’s skin is still translucent, so their blood vessels will be visible. They’re also covered with thin hairs called lanugo, and each of these tiny hairs has a sebaceous gland which produces a substance called vernix caseosa. The lanugo hairs ensure that the vernix caseosa sticks to their skin, and together they protect the baby from the amniotic fluid.

What it’s like for the mum-to-be in week 15

Your body has now adapted as best it can to your pregnancy, and the second trimester is often considered the best of the trimesters. Hopefully, you can now enjoy this exciting time to the full.

Common signs and symptoms


You’ll be doing just fine in week 15, as your hormones put you in such a good mood that you may well feel like grinning from ear to ear. Your thoughts will be determined by this optimism, but also by a certain maternal cautiousness.

It’s still a bit early to be able to feel your baby’s movements, but you’re doing everything you can for their wellbeing. After all, you’ll be thinking about little else other than the tiny person inside you who you’ll soon be cradling in your arms.

Your body changes

From week 15, it will become increasingly clear from your body that you’re expecting a baby, as your bump begins to grow. You’ll keep feeling a pulling sensation in your abdomen or groin as your womb grows. This is nothing to worry about, as long as you don’t experience any other symptoms.

Your body is also building up small amounts of fat that you’ll need later on to produce breast milk.

We recommend a gentle daily massage with high-quality oil to help avoid stretch marks.

Like other parts of your body, you will also notice changes in your breasts. Whether yours are large or small, your hormones will cause them to grow considerably bigger in preparation for breastfeeding. As the skin on your breasts is stretching, a massage with oil will do them the world of good. Your mammary glands are now slowly ramping up production of breast milk, so your breasts may feel tender or sore from time to time – take care when hugging people. A good, supportive maternity bra will be invaluable.

You’ll be putting on enough “padding” as it is, so make sure you eat a healthy and balanced diet, and don’t eat for two! This used to be the advice for expectant mothers, but it quickly leads to excessive weight gain. 255 extra calories a day are enough for pregnant women (roughly the same as a natural yoghurt and a portion of fruit), though of course this can vary from woman to woman. Find out more with our pregnancy weight calculator

It’s often difficult to lose excess weight after you give birth, and being overweight during pregnancy can cause what’s known as gestational diabetes. This would not only be problematic for you, but also for your child. Gestational diabetes means your child will grow to be bigger and heavier than average, so contact your doctor or midwife if you’re worried because they’ll be able to test for it between weeks 24 and 28.

Strengthening your muscles and pelvic floor

If you haven’t done so already, we recommend signing up for a pregnancy class. The instructor will teach you gentle exercises to strengthen your muscles and pelvic floor during the time your bump is growing, as your pelvic floor in particular now has a heavy burden to bear.

Pregnancy classes will also help you to meet other pregnant women, who you can talk to and whose experience you can benefit from, and the instructor will have lots of useful advice and ways to treat the common symptoms of pregnancy.

Top tips

  • Make sure you get enough folic acid every day
  • Make sure you get enough calcium every day
  • Avoid an intake of excessive or empty calories
  • Get a daily “pinching” massage to prevent stretch marks
  • Plan to attend a pregnancy exercise course
  • Get hold of comfy maternity wear
  • Remember to keep making appointments with your gynaecologist or midwife

Questions you may want to ask your doctor or midwife

Pains and pulling sensations in your abdomen

If you suffer pains and pulling sensations in your abdomen for an extended period or experience other distressing symptoms, visit your gynaecologist immediately. If you experience rhythmic, painful spasms or bleeding, a thorough examination is recommended.

Lifting heavy objects

Have you already got a child at home? Then you’ll know all about how much they want to be picked up and carried around. Your midwife can show you the best way to pick up them and other heavy objects without damaging your pelvic floor. Follow their advice in everyday life and you’ll avoid unwelcome symptoms. 

Soothing care for aches and pains

Your mammary glands begin to develop in week 15 so that you can breastfeed your child, and this can lead to unpleasant soreness and heaviness. Midwives can show you various ways to reduce the pain: for instance, there are oils containing useful ingredients that both relieve your skin and help reduce other symptoms caused by your growing womb.

Information about the reviewer:

Marley Hall is a UK-registered, award-winning midwife, educator and author from Surrey. Qualified in 2009, Marley now works as a private independent midwife and is one of the founding members of NowBaby Live. She is a mother of 5 and is passionate about ensuring women and their partners are given informed choices. Find out more here.