Health explainer: Hormonal contraception

They offer a range of information, products and advice aimed at, they say, helping women tune in to their natural cycles (cycle syncing), 'transitioning' off hormonal birth control (hormonal contraception) and dealing with 'post birth control syndrome'.

Some also warn of dire side effects and health impacts from taking hormonal birth control (HBC), including claims of HBC causing infertility, depression and weight gain and even impacting the kinds of partners women using it might choose. What is the truth behind these claims?

There are two main types of hormonal birth control, and several different forms.

The combined contraceptive pill contains forms of oestrogen and progesterone. It works by changing the hormones in the body, so the natural rise in hormones that leads to ovulation (the release of an egg) is stopped.

The progesterone-only (or 'mini' pill) contains only progesterone. It works by thickening the mucus in the cervix, so sperm cannot travel through it. It also changes the lining of the uterus, making it less likely to accept a fertilised egg. It may also prevent ovulation. Progesterone (technically progestin) is also the hormone that is in contraceptive implants, some IUDs and the Depo Provera injection.

Some of the hormone influencers claim that HBC has negatively affected their fertility. But there's no evidence for this, says Sexual Wellbeing Aotearoa Medical Director Dr Beth Messenger. 

"I think there are a couple of things we need to remember", she says. "One is that if you're on the pill for a long time, you are that many years older. Fertility definitely decreases with time. So if you go on the pill at say 15, having never been pregnant, and then come off the pill at 30 and you can't get pregnant… you might blame the pill. But you are now 30, not 15. Fertility is already not at its peak."

Secondly, she points out, "We all assume that we have a high level of fertility. That's why we go on contraception as soon as we start having sex. But in fact, that's not the reality anyway. Sometimes people have a low level of fertility, and they haven't known, because they have very carefully been using birth control all that time.

"The other thing is that we use the pill to manage things like painful periods, and of course one of the causes of painful periods is endometriosis, which in itself causes infertility. So, we can sometimes mask conditions which are also contributing to low fertility by treating them with the pill, when we're not thinking about it in those terms."

Messenger says the evidence says no, but women sometimes say yes.

"The evidence tells us that [depression] is not from that. But I think definitely for individual women it is."

She points out that some women experience worse symptoms - including mood changes - at different times in their cycles even when they're not on HBC, and that everyone is different in terms of how they respond to both natural hormone cycles and HBC. Different pills can have different effects, and a change in pill might mean a change in mood.

"The pills are all synthetic versions of some part of your hormone pathway, which is a massively complicated cascade of hormones. And some of us have some sections of that cascade which are more dominant than others. And it can depend on what bit of their pathway is dominant, as to whether they're going to get on better with this pill or a different one."

"This is an interesting one", Messenger says.

Depo-Provera is the only HBC for which there's evidence of weight gain.

But, she says, again, it's individual. At that level, it looks like HBC might cause weight fluctuations - both ways - in some women. "I've seen women gain weight on birth control, but I've also seen them lose weight".

She points out that many other factors can be at play to cause weight gain.

"We know we naturally gain weight over time. And there's evidence that women also gain weight when they're in relationships."

As outlandish as this sounds, there could be something to it. There have been a few studies looking at women's preferences in a partner and relationship satisfaction while on and off the pill, and they have shown some interesting - and contradictory - things. Some studies have found women prefer more 'masculine' faces when they are not on HBC, and that when they are on HBT they gravitate towards partners with more 'provider' qualities. Women were also found to be more satisfied in their relationships when on the pill, and less satisfied after coming off it.

Messenger thinks this is all; "Absolutely fascinating. And if it's true, it's potentially concerning. But then the suggestion was [also] that at different points in our natural menstrual cycle, we would choose different partners as well, so… that needs to be explored."

Other studies have found no change in women's partner preferences, with or without HBC. As researchers like to say, more research is needed here.

HBC - like all medication - does come with some risks. With the pill, there is a very small chance of blood clots (deep vein thrombosis or pulmonary embolism), heart attacks, and strokes - the risks are higher in people over 35, smokers, people who are overweight or in those who have a family history of blood clots, heart attacks, or strokes.

With most HBC, there is a risk of unscheduled bleeding. This is not usually serious and might change with a change of medication or in the way you take it.

Methods like tracking your cycle can work to prevent pregnancy. But "they require quite a bit of effort," Messenger says. She also warns that cycle-tracking apps are only as good as the information you put into them, and if you are someone with an irregular cycle, they might not be too reliable.

"The app doesn't actually know what's happening in your body right now. It only knows what it's expecting to happen based on the information that you've previously given it. So the more regular your cycle, the more likely it is to be able to have a good guess at whether you are safe or not."

First, do not listen to influencers.

"I'd really like people - rather than just coming off their contraception because they've seen it on TikTok - to actually talk to somebody about what that might mean for them," Messenger says.

"We can explore: are those symptoms they're experiencing really from their birth control? Is it something else? Would a different version of the same thing be better for them? Or a completely different method?"Because if they don't want to be pregnant and they're not using contraception, then we would reasonably assume that at some point they will get pregnant. And then we have to address that pregnancy that they may or may not want to continue."

Also important to know, Messenger says: if you do come off HBC and you do not want to get pregnant, take other precautions straight away.

"Particularly with the combined pill, when you stop it, in the short term you can actually have an increased risk of pregnancy. Ovulation often occurs slightly earlier than you would expect it to, compared to a natural period."

Story written by Niki Bezzant for RNZ.