It’s not uncommon for women to have menstrual cycles that vary in length. One month it might be 28 days — which is considered average — and the next month it could be 31 days, and the next 27. That’s normal.

Menstrual cycles are considered irregular when they fall outside the “normal” range. The Office on Women’s Health reports that an irregular menstrual cycle is one that’s shorter than 21 days or longer than 35.

When counting the days in your cycle, the first day of bleeding is day one, and the last day of the cycle is the first day of bleeding in your next cycle.

It’s possible to become pregnant when you have an irregular menstrual cycle, but you may find it difficult. If you’re unsure about the length of your cycle from month to month, it can be difficult to know when you’re ovulating.

Timing intercourse around ovulation can increase your chances for pregnancy since you’ll need to have sex during your fertile window to conceive. Your fertile window refers to a few days before ovulation and the day you ovulate.

An irregular menstrual cycle may also be a sign of irregular ovulation. You may not ovulate every month or you may ovulate at different times from month to month.

Read on to learn more about irregular periods and pregnancy.

It’s possible to ovulate without later bleeding like a period. This often occurs because of previous uterine scarring or certain hormonal medications.

It’s also possible to have menstrual-like bleeding without ovulation. This generally happens when the uterine lining becomes so thick it becomes unstable and naturally sloughs off.

The uterine lining can become thick without ovulation if the hormone estrogen, which is produced prior to ovulation, continues to be secreted unopposed by the other female hormone, progesterone, which is produced after ovulation.

There are many possible causes for irregular menstruation, and many of the causes can affect ovulation or make getting pregnant more difficult. In some cases, the cause of irregular menstruation is unknown.

Some causes that may affect ovulation and your ability to carry a pregnancy include:

Polycystic ovary syndrome (PCOS)

PCOS is a condition in which the female body secretes too many androgens. Androgens are sometimes thought of as “male” sex hormones. Too many androgens can prevent mature eggs from developing and being released by the fallopian tubes.

PCOS, which affects up to 21 percent of women, is the most common cause of infertility from lack of ovulation. PCOS can be a genetic disorder, but it can also be influenced by lifestyle factors, such as being overweight and sedentary.

Perimenopause

Perimenopause is the time in a woman’s reproductive life when estrogen and progesterone naturally decline. This causes irregular ovulation and periods before they stop altogether, signaling menopause. Typically, perimenopause lasts about four years, but some women can go through it for much longer.

The average age of the onset of perimenopause is 47, with 51 being the average age of the final menstrual period. Perimenopause ends — and menopause begins — when you haven’t had a period for 12 months.

Symptoms of perimenopause may include:

  • hot flashes
  • night sweats
  • moodiness
  • irregular periods

While it’s still possible to become pregnant during perimenopause, it can be more difficult because the released eggs will be older and potentially less viable. You also may not release eggs with every cycle.

Thyroid disease

Your thyroid, which is a small butterfly-shaped organ at the base of your neck, helps regulate hormones that, among other things, impact ovulation and menstruation. In one study, nearly 14 percent of adolescent girls with thyroid disorders also had irregular periods.

Other symptoms of thyroid disease, which include hyperthyroidism and hypothyroidism, can be:

  • mental fuzziness
  • weight changes
  • altered heart and metabolic rates

Weight

Being severely over- or underweight can set off a chain reaction in your body that interrupts hormonal function. That can lead to absent or irregular ovulation, which can also lead to absent or irregular menstruation.

According to research published in BMC Women’s Health, women with a body mass index of less than 20 or greater than 25 were at least 1.1 times more likely to experience menstrual irregularities than women who had BMIs between 20 and 25.

Stress

Stress can impact a wide variety of bodily functions, including ovulation. In one study looking at medical students, those who reported higher levels of perceived stress were more likely to have menstrual irregularities compared with those who didn’t feel highly stressed.

Ovulation generally occurs midway in your cycle. If you have a typical 28-day cycle, you’d ovulate about day 14. But when your periods are irregular, predicting ovulation and timing intercourse to up pregnancy chances can be difficult.

Ovulation predictor kits are fairly accurate at detecting a surge in luteinizing hormone, which triggers ovulation. And while they’re easy to use, requiring just a quick pass through your urine stream, they can get expensive, especially when you’re testing for days or weeks on end.

If you have irregular periods, you may want to wait to use an ovulation predictor kit until you observe other signs of ovulation. Some things to look for:

An increase in cervical mucus

Look for a stretchy, clear, egg white-like discharge on your underwear or when you wipe after using the bathroom. This is a sign that ovulation is near.

A spike in your basal body temperature

Take your basal body temperature with a basal body thermometer first thing in the morning, before you eat, talk, or even get out of bed. Chart your temperature all month long.

When you notice a slight increase, usually half a degree to a whole degree, you may have ovulated. Because this method only shows that ovulation has already occurred, it’s not a good way to predict your fertile window. It may help you understand your body’s typical timing for ovulation in future cycles, though.

See a doctor if:

  • You haven’t had a period for three or more months.
  • You have menstrual bleeding that lasts for more than a week.
  • You’re soaking through a pad or tampon every hour or two, for several hours, during your period.
  • Your periods are very painful.
  • You’ve been trying unsuccessfully to conceive for one year and are younger than 35 or for six months or longer and are 35 or older.

If you’re ovulating, you have the ability to get pregnant, but if you have irregular periods, your chances for pregnancy may be more limited than a woman with regular periods.

The most important thing is to have regular unprotected sex. Aim to have intercourse at least every two to three days.

If you have an underlying medical condition that’s affecting fertility, treating that condition may increase your chances for pregnancy.

Your doctor may prescribe clomiphene citrate (Clomid) to induce ovulation. Clomid has been found to be an effective drug for stimulating ovulation. It has also shown positive results when used in women with PCOS.

Side effects from Clomid may include:

  • hot flashes
  • breast tenderness
  • abdominal bloating
  • release of multiple eggs in one cycle, which can lead to pregnancy with multiples

Weight loss or weight gain may also help. According to the PCOS Awareness Association, losing just 5 to 10 percent of your body weight can help regulate ovulation in women who are overweight.

Get your doctor’s recommendations for gaining or losing weight. They may be able to provide you with meal plans and exercise guidelines, or point you to resources.

If your irregular periods are caused by an underactive or overactive thyroid, your doctor will prescribe drugs that increase the thyroid hormone or block it.

One study published in the found that 35 percent of women with hypothyroidism and infertility who were treated with the drug levothyroxine (Levoxylo, Synthroid, Unithroid) got pregnant versus 26 percent treated with a placebo.

They might, depending on what’s causing your irregular menstruation. If the cause is unknown, you may have no increased risk for pregnancy complications, but you should discuss possible risks with your doctor.

Pregnant women with PCOS are at greater risk for:

Pregnant women with uncontrolled hyperthyroidism have higher rates of delivering a stillborn baby, a premature baby, or a baby with birth defects.

Many women experience irregular periods, mostly due to irregular ovulation. While infrequent ovulation can make it harder to get pregnant, your doctor can help increase your fertility by treating the underlying cause of your irregular menstruation and monitoring your progress once you do conceive. This will help ensure that you have a healthy pregnancy and deliver a healthy baby.

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