You are 18 weeks exactly

Boy or girl? You and your partner may want to start thinking about whether you'd like to find out.

154 days until your due date

Your baby's sex should now be visible on an ultrasound scan, although you may not have one for a couple of weeks yet.

Whether your baby becomes a boy or a girl depends on whether there's a Y chromosome. Boys are XY, and the Y chromosome tells the reproductive glands (gonads) to become testes. These produce testosterone and other hormones that stop female organs developing and trigger normal development of the male genitals.

Without a Y chromosome, the gonad becomes an ovary and the internal genitals develop as female by default; it's the lack of testosterone, not the ovary, that makes the female organs develop. In girls, the uterus forms first and the vagina lengthens upward to meet it.

Explainer

Pink or blue?

Some women wait to buy a full set of baby clothes until they know the sex, but not everyone wants to stick to pink or blue.

Ask a Mum

Our 20-week scan is coming up. I want to know the sex, but my husband doesn't. What should we do?

Disagreements like this can cause tension. I wanted to find out and my partner didn't, so we explained our reasons: I felt knowing would help me prepare emotionally and practically, while he wanted the surprise at the birth.

Talk openly and you'll hopefully reach an agreement. Don't let it get out of hand, and be ready to back down if needed, because it's important to feel united at this special time.

Once you start talking, one of you may not feel as strongly as you thought. You could agree to find out but not tell anyone else. And remember, if you do find out, the result isn't 100 percent accurate.

Time to think about

Gender

Some women feel strongly about whether to find out the baby's sex at the 20-week scan, while others don't mind. If your partner feels strongly, you could follow his lead. Otherwise, ask yourself whether you'd be happy picking two names and getting mostly yellow and green clothes at your shower, or whether knowing the sex would make these tasks more enjoyable.

Explainer

Bigger than the placenta

The umbilical cord is growing thicker and stronger as it carries blood and nutrients to your growing baby, who is now bigger and heavier than the placenta.

This is general information based on NHS guidance, not medical advice, and every pregnancy is different. Talk to your midwife or GP about anything that worries you. If your baby's movements slow down or change, or you have bleeding, severe pain or leaking fluid, call your maternity unit straight away. In an emergency, call 999.